code: def custom_callback(self, view_func):
          
          @wraps(view_func)
          def decorated(*args, **kwargs):
              plainreturn, data = self._process_callback('custom')
              if plainreturn:
                  return data
              else:
                  return view_func(data, *args, **kwargs)
          self._custom_callback = decorated
          return decorated
docs: Wrapper function to use a custom callback.
          The custom OIDC callback will get the custom state field passed in with
          redirect_to_auth_server.