[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.

[crberube]

The webhook approach sounds interesting because that would enable flexibility for many different use cases / notification channels.

[github-actions[bot]]

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