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)

data/registry_model/registry_oci_model.py

@@ -482,6 +482,22 @@ class OCIModel(SharedModel, RegistryDataInterface):
     retriever = RepositoryContentRetriever(manifest_row.repository_id, storage)
     return parsed.get_schema1_manifest(namespace_name, repo_name, tag_name, retriever)
 
+  def convert_manifest(self, manifest, namespace_name, repo_name, tag_name, allowed_mediatypes,
+                       storage):
+    try:
+      parsed = manifest.get_parsed_manifest()
+    except ManifestException:
+      return None
+
+    try:
+      manifest_row = database.Manifest.get(id=manifest._db_id)
+    except database.Manifest.DoesNotExist:
+      return None
+
+    retriever = RepositoryContentRetriever(manifest_row.repository_id, storage)
+    return parsed.convert_manifest(allowed_mediatypes, namespace_name, repo_name, tag_name,
+                                   retriever)
+
   def create_manifest_with_temp_tag(self, repository_ref, manifest_interface_instance,
                                     expiration_sec, storage):
     """ Creates a manifest under the repository and sets a temporary tag to point to it.