Add self approval of merge requests setting to Merge Request Approvals API

What does this MR do?

Add self approval of merge requests setting to Merge Request Approvals API. The setting is exposed in API responses and can also be updated.

What are the relevant issue numbers?

• #9496 (closed)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated via this MR
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the style guides
• EE specific content should be in the top level /ee folder
• For a paid feature, have we considered GitLab.com plans, how it works for groups, and is there a design for promoting it to users who aren't on the correct plan?
• Security reports checked/validated by reviewer

Closes #9496 (closed)