GraphQL API query and service for suggested reviewers

What does this MR do and why?

GraphQL API to query suggested reviewers

• Currently "root" is returned but actual "suggested_reviewers" will come from a database cache column (merge_requests.suggested_reviewer"), when the background job computes it Merge request suggested reviewers fetch service (!97533 - merged)
• Behind a disabled feature flag and will be launched so.

Related to #369591 (closed)

Screenshots or screen recordings

How to set up and validate locally

1. Go to http://gdk.test:3000/-/graphql-explorer
2. See

{
  mergeRequest(id: "gid://gitlab/MergeRequest/1") {
    id
    iid
    projectId
    suggestedReviewers {
      version
      reviewers
    }
  }
}

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.