cloudapi: implement GCP target results and include UploadOptions

Return GCP-specific target results form the worker, similar as it is
done for AWS.

Extend Cloud API to allow GCP-specific upload Options.

Modify Cloud API to return UploadOptions as part of the UploadStatus.

Modify Cloud API integration test to check returned upload Options and
upload Type.

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

internal/target/targetresult.go

@@ -47,6 +47,8 @@ func UnmarshalTargetResultOptions(trName string, rawOptions json.RawMessage) (Ta
 	switch trName {
 	case "org.osbuild.aws":
 		options = new(AWSTargetResultOptions)
+	case "org.osbuild.gcp":
+		options = new(GCPTargetResultOptions)
 	default:
 		return nil, fmt.Errorf("Unexpected target result name: %s", trName)
 	}