container: add support for uploading to registries

Add a new generic container registry client via a new `container`
package. Use this to create a command line utility as well as a
new upload target for container registries.

The code uses the github.com/containers/* project and packages to
interact with container registires that is also used by skopeo,
podman et al. One if the dependencies is `proglottis/gpgme` that
is using cgo to bind libgpgme, so we have to add the corresponding
devel package to the BuildRequires as well as installing it on CI.

Checks will follow later via an integration test.

.github/workflows/tests.yml

@@ -20,8 +20,9 @@ jobs:
     steps:
         # krb5-devel is needed to test internal/upload/koji package
         # gcc is needed to build the mock dnf-json binary for the unit tests
+        # gpgme-devel is needed for container upload dependencies
       - name: Install build and test dependencies
-        run: dnf -y install krb5-devel gcc git-core go
+        run: dnf -y install krb5-devel gcc git-core go gpgme-devel
 
       - name: Check out code into the Go module directory
         uses: actions/checkout@v3
@@ -79,6 +80,10 @@ jobs:
       - name: Install kerberos devel package
         run: sudo apt-get install -y libkrb5-dev
 
+      # This is needed for the container upload dependencies
+      - name: Install libgpgme devel package
+        run: sudo apt-get install -y libgpgme-dev
+
       - name: Run golangci-lint
         run: $(go env GOPATH)/bin/golangci-lint run --timeout 5m0s