Create "Approve Merge Request" as Separate Permission

Background

Customers are looking for separation of duties when it comes to approving merge requests. They only want certain users to be able to do this.

This is more complex than other granular permission requests because it is part of the approval rule configuration.

More details here.

This request is a blocker for some security related permissions requests. One role would require the following:

1. Change vulnerability status - admin_*
2. Approve Merge Requests - admin_*, currently blocked, hence the creation of this issue
3. View Security Dashboard - read_*
4. View Vulnerability Report - read_*, already done
5. View Dependency List - read_*

Proposal

Add Approve Merge Requests as a permission that can be configurable based on the customizable roles framework.