[feat] add notification channel for flag updates

[zhouzhuojie]

Teams need more visibility into how and when flags are updated. For example, Email or Slack notifications can be good channels.

Not sure if this feature is a good addon to the current Flagr setup. Maybe we can build it in Flagr, maybe we can expose Flagr webhooks and let other applications implement it.

[gedw99]

Also it would be good if I can get a web hook or similar to then control apdating users , apps or anything else I want.

the web hook just needs to have a decent amount of content so I can determine which users are effected so that I can then alert them.

I don’t know how flagr does this under the hood with PostgreSQL, but I imagine the outbox pattern would be a good way if it does not already use this

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