Add support for pushing and pulling schema 2 manifests with remote layers

This is required for windows image support

workers/manifestbackfillworker.py

@@ -63,6 +63,10 @@ class BrokenManifest(ManifestInterface):
   def blob_digests(self):
     return []
   
+  @property
+  def local_blob_digests(self):
+    return []
+
   def child_manifests(self, lookup_manifest_fn):
     return None