API to list external approval rules for a merge request

Why are we doing this work

To allow frontend engineers to implement the UI to show on a merge request the state of external approval rules, we need to expose an API as described below:

Request

[GET] /projects/:id/merge_requests/:merge_request_iid/status_checks

Response

{
  status_checks: [
    {
      name: "Something rule",
      external_url: "http://something.org",
      status: "pending|approved"
    },
    ....
  ]
}

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Sync video discussion between @mwoolf and @rob.hunt: https://www.youtube.com/watch?v=-DfmxFDeJj8