Implement the new OCI-based registry data model

Note that this change does *not* enable the new data model by default, but does allow it to be used when a special environment variable is specified.

endpoints/api/tag.py

@@ -217,6 +217,7 @@ class RepositoryTagImages(RepositoryParamResource):
 
     all_images = None
     if parsed_args['owned']:
+      # TODO(jschorr): Remove the `owned` image concept once we are fully on V2_2.
       all_images = registry_model.get_legacy_images_owned_by_tag(tag_ref)
     else:
       image_with_parents = registry_model.get_legacy_image(repo_ref, image_id, include_parents=True)