Move manifest backfill for V1 tags into the new registry model interface

data/registry_model/interface.py

@@ -122,3 +122,12 @@ class RegistryDataInterface(object):
   @abstractmethod
   def get_security_status(self, manifest_or_legacy_image):
     """ Returns the security status for the given manifest or legacy image or None if none. """
+
+  @abstractmethod
+  def backfill_manifest_for_tag(self, tag):
+    """ Backfills a manifest for the V1 tag specified.
+        If a manifest already exists for the tag, returns that manifest.
+
+        NOTE: This method will only be necessary until we've completed the backfill, at which point
+        it should be removed.
+    """