Support global hooks

[banto6]

Note: if you have a question or want discussion please post in the discussions area.

Is your feature request related to a problem? Please describe.

Currently, implementing the same notification method for each repository or plan can only be configured individually within the corresponding plan or repository.

Describe the solution you'd like

Add the functionality to maintain global Hooks, and then reference the corresponding global Hooks in plans or repositories.

Additional context

[garethgeorge]

The are architectural reasons that this probably isn't something that makes sense for backrest -- though I do agree that backrest needs a good way to share configuration between repos.

I think some sort of templating framework would be a good UI feature and is the most likely solution to this problem, though unsure when I'll budget time for it as it introduces significant UI complexity. In the meantime, editing the config.json and directly copying hooks between repos can be a pretty good way to quickly configure repos in bulk.

[caco3]

Please have a look on #1324 where I added some improvement for this issue. It does not provide global hooks, but the config of an existing hook can now easier be re-used.