Apply decorator to authenticate the user who is making the request.
        :param view_func: The flask view func.

def _apply_auth(self, view_func):
        """
        Apply decorator to authenticate the user who is making the request.
        :param view_func: The flask view func.
        """
        @functools.wraps(view_func)
        def decorated(*args, **kwargs):
            if not self.auth:
                return view_func(*args, **kwargs)

            auth_data = self.auth.get_authorization()

            if auth_data is None:
                return self._handle_authentication_error()

            if not self._authentication_callback or not self._authentication_callback(auth_data):
                return self._handle_authentication_error()

            return view_func(*args, **kwargs)

        return decorated