[Backend] feature_flags api 增加返回 user_list 数据

feature flag api 增加返回 user_list 数据

https://docs.gitlab.cn/jh/api/feature_flags.html#%E9%A1%B9%E7%9B%AE%E7%9A%84%E5%88%97%E8%A1%A8%E5%8A%9F%E8%83%BD%E6%A0%87%E5%BF%97

upstream issue: https://gitlab.com/gitlab-org/gitlab/-/issues/417984