Add CI job to watch upstream feature flgas folder changes

related issue #798 (closed)

1. only run on main-jh branch and JH_SYNC_TOKEN triggered, to make sure run this job only when sync code
2. this job allow failure
3. get upstream_latest_commit from config/feature_flags
4. get JH_know_latest_commit from CI cache
5. check JH_know_latest_commit equal upstream_latest_commit or not
6. if equal, do nothing
7. if not equal
   1. send slack message to notify dev
   2. create jihulab issue for tracking
   3. save upstream_latest_commit as JH_know_latest_commit in CI cache

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处理 feature flags 变化流程（考虑放到文档中）

1. 极狐 CI job "watch-upstream-feature-flags" 会监控 upstream 中 config/feature_flags 文件夹的变化
2. 发现 upstream 有变化，会在 gitlab project 中创建 issue，方便 dev 记录排查信息，该 issue 会带上 featureflag::changed label
3. 另外会发送 slack 消息到 #feature-flag-watcher channel 中，通知 dev
4. dev 收到消息后开始排查相关变化，必要时需要通知 SRE 同事
5. dev 或者 SRE 处理完成后，需要关闭第 2 步创建的 issue，同时在 #feature-flag-watcher channel 中对消息回复"完成"表情，表明已经处理完成