Add Prevent approval of merge requests by commiter to the API

In 11.10 the approver interface has been redesigned but two features are not available in the API.

• Prevent approval of merge requests by merge request committers
• Prevent approval of merge requests by merge request author https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/12721

I have seen quite a few requests if these features can be added to the API call.