Enhancements for Docker schema implementations in preparing for supporting schema 2 in the OCI model

This adds additional required properties and methods to the Docker schema interface to allow us to treat both schema1 and schema2 manifests and lists logically equivalent from the OCI mode perspective

workers/manifestbackfillworker.py

@@ -62,6 +62,18 @@ class BrokenManifest(ManifestInterface):
   @property
   def blob_digests(self):
     return []
+  
+  def child_manifests(self, lookup_manifest_fn):
+    return None
+
+  def get_manifest_labels(self, lookup_config_fn):
+    return {}
+
+  def unsigned(self):
+    return self
+
+  def generate_legacy_layers(self, images_map, lookup_config_fn):
+    return None
 
 
 class ManifestBackfillWorker(Worker):