Add authentication to webhooks

[mrPsycho]

Is your feature request related to a problem? Please describe.
In addition to #1242 it would be great to have authentication option for webhooks.
or, maybe, just whitelist.

Describe the solution you'd like
would be great to use X-Gitlab-Token for validation

[nijel]

I was thinking about that as well. The token would have to be stored per project though to avoid leaking it across projects on shared instances.

[vladox]

@nijel do you accept PR regarding this feature?

[nijel]

Patches are always welcome :-). It just have to be opt-in feature to avoid breaking existing setups.

[github-actions]

This issue has been added to the backlog. It is not scheduled on our roadmap, but it eventually might be implemented. In case you desperately need this feature, please consider helping or funding the development.

[nijel]

This doesn't have a high priority as Weblate doesn't use much from the payload - at most it can trigger matching repository update based on the payload.