main: add new --registrations options

This new flag allows to add a file with registration data. This
is meant to eventually hold all sort of registrations like
ansible or satelite but initially only contains the redhat
subscription. Currently only JSON is supported.

It looks like:
```json:
{
  "redhat": {
    "subscription": {
      "activation_key": "ak_123",
      "organization": "org_123",
      "server_url": "server_url_123",
      "base_url": "base_url_123",
      "insights": true,
      "rhc": true,
      "proxy": "proxy_123"
    }
  }
}
```

This is not part of the blueprint (today) because its more
ephemeral than the things we usually put into the blueprint.

This allows us to build images that are immediately registered. It
also keeps our options open in the future. If we move to a new
blueprint format where we support multiple blueprints and also
ephemeral data like this the "registrations" flag just becomes an
alias for "--blueprint".

cmd/image-builder/main.go

@@ -2,6 +2,7 @@ package main
 
 import (
 	"bytes"
+	"encoding/json"
 	"errors"
 	"fmt"
 	"io"
@@ -17,6 +18,7 @@ import (
 
 	"github.com/osbuild/image-builder-cli/pkg/progress"
 	"github.com/osbuild/images/pkg/arch"
+	"github.com/osbuild/images/pkg/customizations/subscription"
 	"github.com/osbuild/images/pkg/imagefilter"
 	"github.com/osbuild/images/pkg/osbuild"
 	"github.com/osbuild/images/pkg/ostree"
@@ -97,6 +99,37 @@ func ostreeImageOptions(cmd *cobra.Command) (*ostree.ImageOptions, error) {
 	}, nil
 }
 
+type registrations struct {
+	Redhat struct {
+		Subscription *subscription.ImageOptions `json:"subscription,omitempty"`
+	} `json:"redhat,omitempty"`
+}
+
+func subscriptionImageOptions(cmd *cobra.Command) (*subscription.ImageOptions, error) {
+	regFilePath, err := cmd.Flags().GetString("registrations")
+	if err != nil {
+		return nil, err
+	}
+	if regFilePath == "" {
+		return nil, nil
+	}
+
+	f, err := os.Open(regFilePath)
+	if err != nil {
+		return nil, fmt.Errorf("cannot open registrations file: %w", err)
+	}
+	defer f.Close()
+
+	// XXX: support yaml eventually
+	var regs registrations
+	dec := json.NewDecoder(f)
+	dec.DisallowUnknownFields()
+	if err := dec.Decode(&regs); err != nil {
+		return nil, fmt.Errorf("cannot parse registrations file: %w", err)
+	}
+	return regs.Redhat.Subscription, nil
+}
+
 type cmdManifestWrapperOptions struct {
 	useBootstrapIfNeeded bool
 }
@@ -160,6 +193,10 @@ func cmdManifestWrapper(pbar progress.ProgressBar, cmd *cobra.Command, args []st
 		}
 		customSeed = &seedFlagVal
 	}
+	subscription, err := subscriptionImageOptions(cmd)
+	if err != nil {
+		return nil, err
+	}
 	// no error check here as this is (deliberately) not defined on
 	// "manifest" (if "images" learn to set the output filename in
 	// manifests we would change this
@@ -199,6 +236,7 @@ func cmdManifestWrapper(pbar progress.ProgressBar, cmd *cobra.Command, args []st
 		RpmDownloader:  rpmDownloader,
 		WithSBOM:       withSBOM,
 		CustomSeed:     customSeed,
+		Subscription:   subscription,
 
 		ForceRepos: forceRepos,
 	}
@@ -407,7 +445,7 @@ operating systems like Fedora, CentOS and RHEL with easy customizations support.
 		RunE:         cmdListImages,
 		SilenceUsage: true,
 		Args:         cobra.NoArgs,
-        Aliases:      []string{"list-images"},
+		Aliases:      []string{"list-images"},
 	}
 	listCmd.Flags().StringArray("filter", nil, `Filter distributions by a specific criteria (e.g. "type:iot*")`)
 	listCmd.Flags().String("format", "", "Output in a specific format (text, json)")
@@ -430,6 +468,7 @@ operating systems like Fedora, CentOS and RHEL with easy customizations support.
 	manifestCmd.Flags().String("ostree-url", "", `OSTREE url`)
 	manifestCmd.Flags().Bool("use-librepo", true, `use librepo to download packages (disable if you use old versions of osbuild)`)
 	manifestCmd.Flags().Bool("with-sbom", false, `export SPDX SBOM document`)
+	manifestCmd.Flags().String("registrations", "", `filename of a registrations file with e.g. subscription details`)
 	rootCmd.AddCommand(manifestCmd)
 
 	uploadCmd := &cobra.Command{