internal/upload: Add support for upload to GCP and CLI tool using it

Add new internal upload target for Google Cloud Platform and
osbuild-upload-gcp CLI tool which uses the API.

Supported features are:
- Authenticate with GCP using explicitly provided JSON credentials
  file or let the authentication be handled automatically by the
  Google cloud client library. The later is useful e.g. when the worker
  is running in GCP VM instance, which has associated permissions with
  it.
- Upload an existing image file into existing Storage bucket.
- Verify MD5 checksum of the uploaded image file against the local
  file's checksum.
- Import the uploaded image file into Compute Node as an Image.
- Delete the uploaded image file after a successful image import.
- Delete all cache files from storage created as part of the image
  import build job.
- Share the imported image with a list of specified accounts.

GCP-specific image type is not yet added, since GCP supports importing
VMDK and VHD images, which the osbuild-composer already supports.

Update go.mod, vendor/ content and SPEC file with new dependencies.

Signed-off-by: Tomas Hozza <thozza@redhat.com>

vendor/cloud.google.com/go/storage/doc.go

@@ -0,0 +1,218 @@
+// Copyright 2016 Google LLC
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+/*
+Package storage provides an easy way to work with Google Cloud Storage.
+Google Cloud Storage stores data in named objects, which are grouped into buckets.
+
+More information about Google Cloud Storage is available at
+https://cloud.google.com/storage/docs.
+
+See https://godoc.org/cloud.google.com/go for authentication, timeouts,
+connection pooling and similar aspects of this package.
+
+All of the methods of this package use exponential backoff to retry calls that fail
+with certain errors, as described in
+https://cloud.google.com/storage/docs/exponential-backoff. Retrying continues
+indefinitely unless the controlling context is canceled or the client is closed. See
+context.WithTimeout and context.WithCancel.
+
+
+Creating a Client
+
+To start working with this package, create a client:
+
+    ctx := context.Background()
+    client, err := storage.NewClient(ctx)
+    if err != nil {
+        // TODO: Handle error.
+    }
+
+The client will use your default application credentials.
+
+If you only wish to access public data, you can create
+an unauthenticated client with
+
+    client, err := storage.NewClient(ctx, option.WithoutAuthentication())
+
+Buckets
+
+A Google Cloud Storage bucket is a collection of objects. To work with a
+bucket, make a bucket handle:
+
+    bkt := client.Bucket(bucketName)
+
+A handle is a reference to a bucket. You can have a handle even if the
+bucket doesn't exist yet. To create a bucket in Google Cloud Storage,
+call Create on the handle:
+
+    if err := bkt.Create(ctx, projectID, nil); err != nil {
+        // TODO: Handle error.
+    }
+
+Note that although buckets are associated with projects, bucket names are
+global across all projects.
+
+Each bucket has associated metadata, represented in this package by
+BucketAttrs. The third argument to BucketHandle.Create allows you to set
+the initial BucketAttrs of a bucket. To retrieve a bucket's attributes, use
+Attrs:
+
+    attrs, err := bkt.Attrs(ctx)
+    if err != nil {
+        // TODO: Handle error.
+    }
+    fmt.Printf("bucket %s, created at %s, is located in %s with storage class %s\n",
+        attrs.Name, attrs.Created, attrs.Location, attrs.StorageClass)
+
+Objects
+
+An object holds arbitrary data as a sequence of bytes, like a file. You
+refer to objects using a handle, just as with buckets, but unlike buckets
+you don't explicitly create an object. Instead, the first time you write
+to an object it will be created. You can use the standard Go io.Reader
+and io.Writer interfaces to read and write object data:
+
+    obj := bkt.Object("data")
+    // Write something to obj.
+    // w implements io.Writer.
+    w := obj.NewWriter(ctx)
+    // Write some text to obj. This will either create the object or overwrite whatever is there already.
+    if _, err := fmt.Fprintf(w, "This object contains text.\n"); err != nil {
+        // TODO: Handle error.
+    }
+    // Close, just like writing a file.
+    if err := w.Close(); err != nil {
+        // TODO: Handle error.
+    }
+
+    // Read it back.
+    r, err := obj.NewReader(ctx)
+    if err != nil {
+        // TODO: Handle error.
+    }
+    defer r.Close()
+    if _, err := io.Copy(os.Stdout, r); err != nil {
+        // TODO: Handle error.
+    }
+    // Prints "This object contains text."
+
+Objects also have attributes, which you can fetch with Attrs:
+
+    objAttrs, err := obj.Attrs(ctx)
+    if err != nil {
+        // TODO: Handle error.
+    }
+    fmt.Printf("object %s has size %d and can be read using %s\n",
+        objAttrs.Name, objAttrs.Size, objAttrs.MediaLink)
+
+Listing objects
+
+Listing objects in a bucket is done with the Bucket.Objects method:
+
+    query := &storage.Query{Prefix: ""}
+
+    var names []string
+    it := bkt.Objects(ctx, query)
+    for {
+        attrs, err := it.Next()
+        if err == iterator.Done {
+            break
+        }
+        if err != nil {
+            log.Fatal(err)
+        }
+        names = append(names, attrs.Name)
+    }
+
+If only a subset of object attributes is needed when listing, specifying this
+subset using Query.SetAttrSelection may speed up the listing process:
+
+    query := &storage.Query{Prefix: ""}
+    query.SetAttrSelection([]string{"Name"})
+
+    // ... as before
+
+ACLs
+
+Both objects and buckets have ACLs (Access Control Lists). An ACL is a list of
+ACLRules, each of which specifies the role of a user, group or project. ACLs
+are suitable for fine-grained control, but you may prefer using IAM to control
+access at the project level (see
+https://cloud.google.com/storage/docs/access-control/iam).
+
+To list the ACLs of a bucket or object, obtain an ACLHandle and call its List method:
+
+    acls, err := obj.ACL().List(ctx)
+    if err != nil {
+        // TODO: Handle error.
+    }
+    for _, rule := range acls {
+        fmt.Printf("%s has role %s\n", rule.Entity, rule.Role)
+    }
+
+You can also set and delete ACLs.
+
+Conditions
+
+Every object has a generation and a metageneration. The generation changes
+whenever the content changes, and the metageneration changes whenever the
+metadata changes. Conditions let you check these values before an operation;
+the operation only executes if the conditions match. You can use conditions to
+prevent race conditions in read-modify-write operations.
+
+For example, say you've read an object's metadata into objAttrs. Now
+you want to write to that object, but only if its contents haven't changed
+since you read it. Here is how to express that:
+
+    w = obj.If(storage.Conditions{GenerationMatch: objAttrs.Generation}).NewWriter(ctx)
+    // Proceed with writing as above.
+
+Signed URLs
+
+You can obtain a URL that lets anyone read or write an object for a limited time.
+You don't need to create a client to do this. See the documentation of
+SignedURL for details.
+
+    url, err := storage.SignedURL(bucketName, "shared-object", opts)
+    if err != nil {
+        // TODO: Handle error.
+    }
+    fmt.Println(url)
+
+Post Policy V4 Signed Request
+
+A type of signed request that allows uploads through HTML forms directly to Cloud Storage with
+temporary permission. Conditions can be applied to restrict how the HTML form is used and exercised
+by a user.
+
+For more information, please see https://cloud.google.com/storage/docs/xml-api/post-object as well
+as the documentation of GenerateSignedPostPolicyV4.
+
+    pv4, err := storage.GenerateSignedPostPolicyV4(bucketName, objectName, opts)
+    if err != nil {
+        // TODO: Handle error.
+    }
+    fmt.Printf("URL: %s\nFields; %v\n", pv4.URL, pv4.Fields)
+
+Errors
+
+Errors returned by this client are often of the type [`googleapi.Error`](https://godoc.org/google.golang.org/api/googleapi#Error).
+These errors can be introspected for more information by type asserting to the richer `googleapi.Error` type. For example:
+
+	if e, ok := err.(*googleapi.Error); ok {
+		  if e.Code == 409 { ... }
+	}
+*/
+package storage // import "cloud.google.com/go/storage"