Allow triggering of project test webhooks via the API

Problem to solve

There used to be a way to trigger a test on webhook through a call to the GUI on <gitlabURL>/<project>/hooks/<hookID>/test.

This can't be done now because of CRSF protection and a way to trigger a project's hook from API does not seems to exists.

On some setup, calling the webhooks directly may be a no-go at network level and, as a bonus, Gitlab provide also a level of authentication.

Proposal

Add an API way to trigger a project's webhooks next to other hooks operations (https://docs.gitlab.com/ee/api/projects.html#hooks) and also group webhooks (https://docs.gitlab.com/ee/api/groups.html#hooks). Issue for adding test for group webhooks: Allow triggering group test webhooks via API (#455589 - closed).

What does success look like, and how can we measure that?

Project and group webhook can be tested through an API call. Issue for adding test for group webhooks: Allow triggering group test webhooks via API (#455589 - closed).