``code_challenge`` has been provided.

        See ``.get_code_challenge``.

        Must return ``plain`` or ``S256``. You can return a custom value if you have
        implemented your own ``AuthorizationCodeGrant`` class.

        :param code: Authorization code.
        :param request: OAuthlib request.
        :type request: oauthlib.common.Request
        :rtype: code_challenge_method string

        Method is used by:
            - Authorization Code Grant - when PKCE is active

        r