Return a serializer for the session that also signs data.

        This will return None if the app is not configured for secrets.

def get_signing_serializer(self, app: 'Quart') -> Optional[URLSafeTimedSerializer]:
        """Return a serializer for the session that also signs data.

        This will return None if the app is not configured for secrets.
        """
        if not app.secret_key:
            return None

        options = {
            'key_derivation': self.key_derivation,
            'digest_method': self.digest_method,
        }
        return URLSafeTimedSerializer(
            app.secret_key, salt=self.salt, serializer=self.serializer, signer_kwargs=options,
        )