osbuild: add manifest-lists input to skopeo stage

The skopeo stage in osbuild supports an second optional set of inputs
called `manifest-lists`.  This is an array of files, i.e.,
`org.osbuild.files` type input.

To support this we need a new type for the skopeo stage inputs that can
encompass both input types, images and manifest-lists.

internal/manifest/os.go

@@ -352,7 +352,7 @@ func (p *OS) serialize() osbuild.Pipeline {
 			pipeline.AddStage(osbuild.NewContainersStorageConfStage(containerStoreOpts))
 		}
 
-		skopeo := osbuild.NewSkopeoStage(storagePath, images)
+		skopeo := osbuild.NewSkopeoStage(storagePath, images, nil)
 		pipeline.AddStage(skopeo)
 	}
 

internal/osbuild/skopeo_stage.go

@@ -12,10 +12,18 @@ type SkopeoStageOptions struct {
 
 func (o SkopeoStageOptions) isStageOptions() {}
 
-func NewSkopeoStage(path string, images ContainersInput) *Stage {
+type SkopeoStageInputs struct {
+	Images        ContainersInput `json:"images"`
+	ManifestLists *FilesInput     `json:"manifest-lists,omitempty"`
+}
 
-	inputs := ContainersInputs{
-		"images": images,
+func (SkopeoStageInputs) isStageInputs() {}
+
+func NewSkopeoStage(path string, images ContainersInput, manifests *FilesInput) *Stage {
+
+	inputs := SkopeoStageInputs{
+		Images:        images,
+		ManifestLists: manifests,
 	}
 
 	return &Stage{