Given a lambda_name and stage_name, return a valid API URL.

def get_api_url(self, lambda_name, stage_name):
        """
        Given a lambda_name and stage_name, return a valid API URL.
        """
        api_id = self.get_api_id(lambda_name)
        if api_id:
            return "https://{}.execute-api.{}.amazonaws.com/{}".format(api_id, self.boto_session.region_name, stage_name)
        else:
            return None