Returns an `rdf_crypto.Hash` instance for given URN of an AFF4 file.

def GetUrnHashEntry(urn, token=None):
  """Returns an `rdf_crypto.Hash` instance for given URN of an AFF4 file."""
  if data_store.RelationalDBEnabled():
    client_id, vfs_path = urn.Split(2)
    path_type, components = rdf_objects.ParseCategorizedPath(vfs_path)

    path_info = data_store.REL_DB.ReadPathInfo(client_id, path_type, components)
    return path_info.hash_entry
  else:
    with aff4.FACTORY.Open(urn, token=token) as fd:
      return GetFileHashEntry(fd)