Use `push` web hook events for pull mirroring
Many git servers support making a HTTP POST request to an endpoint for each
push event they receive. Rather than polling for pull mirroring, this can be triggered based on a webhook.
- Add API endpoint to receive webhook from other GitLab instances, GitHub, Bitbucket etc.
Links / references
Required to improve performance of pull mirroring, which is half of bi-directional sync: #3745
What is it? Why should someone use this feature? What is the underlying (business) problem? How do you use this feature?
Who is this for? Provide one or more use cases.
Make sure these are completed before closing the issue, with a link to the relevant commit.