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

Problem to solve

In the Merge Request Approvals UI (accessed via Project Settings -> General -> Merge request approvals), there is a setting for Enable self approval of merge requests, which is described in the documentation and was added in 11.3.

While this setting is available in the UI, it is not currently supported in the Merge Request Approvals API. This feature proposal is to support this setting in the API.

Proposed by customer in ticket: https://gitlab.zendesk.com/agent/tickets/112914 (internal use only)

Target audience

Delaney, Development Team Lead, https://design.gitlab.com/research/personas#persona-delaney
Sasha, Software Developer, https://design.gitlab.com/research/personas#persona-sasha

Customer would like to get and edit this value via the API.

Further details

This setting is supported in the UI for Merge Request Approvals so it should also be supported in the API, for both the GET /projects/:id/approvals and POST /projects/:id/approvals calls.

Proposal

Add support for the Enable self approval of merge requests for the Merge Request Approvals API.

What does success look like, and how can we measure that?