add boilerplate for verbs data interface

data/interfaces/verbs.py

@@ -0,0 +1,18 @@
+class VerbsDataInterface(object):
+  """
+  Interface that represents all data store interactions required by the registry's custom HTTP
+  verbs.
+  """
+  @classmethod
+  def repository_is_public(cls, namespace_name, repo_name):
+    """
+    Returns a boolean for whether the repository with the given name and namespace is public.
+    """
+    raise NotImplementedError()
+
+
+class PreOCIModel(VerbsDataInterface):
+  """
+  PreOCIModel implements the data model for the registry's custom HTTP verbs using a database schema
+  before it was changed to support the OCI specification.
+  """