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_pre_oci_model.py

@@ -549,6 +549,18 @@ class PreOCIModel(SharedModel, RegistryDataInterface):
     except ManifestException:
       return None
 
+  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:
+      return parsed.convert_manifest(allowed_mediatypes, namespace_name, repo_name, tag_name, None)
+    except ManifestException:
+      return None
+
   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.