Support pulling of schema2 manifests directly via a manifest list tag

This change ensures that if a manifest list is requested with an accepts header for a *schema 2* manifest, the legacy manifest (if any) is returned as schema 2 if it was pushed as a schema 2 manifest (rather than being auto-converted to schema 1)

workers/manifestbackfillworker.py

@@ -98,6 +98,9 @@ class BrokenManifest(ManifestInterface):
   def get_requires_empty_layer_blob(self, content_retriever):
     return False
 
+  def convert_manifest(self, media_types, namespace_name, repo_name, tag_name, lookup_fn):
+    return None
+
 
 class ManifestBackfillWorker(Worker):
   def __init__(self):