Merge request suggested reviewers fetch service

What does this MR do and why?

Optimize suggested reviewers service

Related to #373173 (closed)

The changes are behind a a user facing feature flag (:suggested_reviewers)

How to set up and validate locally

MergeRequests::FetchSuggestedReviewersService.new.execute(MergeRequest.last)

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

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