code: def register(self, identified_with, identifier, user):
          
          self.kv_store.set(
              self._get_storage_key(identified_with, identifier),
              self.serialization.dumps(user).encode(),
          )
docs: Register new key for given client identifier.
  
          This is only a helper method that allows to register new
          user objects for client identities (keys, tokens, addresses etc.).
  
          Args:
              identified_with (object): authentication middleware used
                  to identify the user.
              identifier (str): user identifier.
              user (str): user object to be stored in the backend.