Approve and remove approval of merge request via API

Description

• Approve merge request via API.
• Remove approval via API.
• Need to design how to approve / remove approval a merge request on behalf of an eligible approver.
• Validations.
• System notes.

Original description

We would like to be able to approve a merge request using the GitLab API. This would mean that we could automatically approve a merge request when the automated tests succeed.