Add custom permission to `Approve Merge Requests`

What does this MR do and why?

This change makes it possible to define a custom role with the permission to approve a merge request.

#412708 (closed)

How to set up and validate locally

1. create a user maintainer
2. create a user developer
3. create a new project project a
4. add maintainer user to project a via a custom role with the admin_merge_request permission.
5. add developer user to project a via project membership permissions.
6. submit a merge request to project a with developer as the author.
7. approve the merge request as the maintainer user.

Additional instructions can be found here.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.