Call create rich menu API.

        https://developers.line.me/en/docs/messaging-api/reference/#create-rich-menu

        :param rich_menu: Inquired to create a rich menu object.
        :type rich_menu: T <= :py:class:`linebot.models.rich_menu.RichMenu`
        :param timeout: (optional) How long to wait for the server
            to send data before giving up, as a float,
            or a (connect timeout, read timeout) float tuple.
            Default is self.http_client.timeout
        :type timeout: float | tuple(float, float)
        :rtype: str
        :return: rich menu id

def create_rich_menu(self, rich_menu, timeout=None):
        """Call create rich menu API.

        https://developers.line.me/en/docs/messaging-api/reference/#create-rich-menu

        :param rich_menu: Inquired to create a rich menu object.
        :type rich_menu: T <= :py:class:`linebot.models.rich_menu.RichMenu`
        :param timeout: (optional) How long to wait for the server
            to send data before giving up, as a float,
            or a (connect timeout, read timeout) float tuple.
            Default is self.http_client.timeout
        :type timeout: float | tuple(float, float)
        :rtype: str
        :return: rich menu id
        """
        response = self._post(
            '/v2/bot/richmenu', data=rich_menu.as_json_string(), timeout=timeout
        )

        return response.json.get('richMenuId')