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.

Proposal

• 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: https://gitlab.com/gitlab-org/gitlab-ee/issues/3745

Documentation blurb

Overview

What is it? Why should someone use this feature? What is the underlying (business) problem? How do you use this feature?

Use cases

Who is this for? Provide one or more use cases.

Feature checklist

Make sure these are completed before closing the issue, with a link to the relevant commit.

• Feature assurance
• Documentation
• Added to features.yml