Adding "approver" to merge request resolver

What does this MR do and why?

This adds the backend portion of the "approver" filter for the upcoming Vue enhancement: Convert merge request listing page to Vue (&10827)

This is basically the EE only version of Add several backend filters to the Vue version ... (!160938 - merged) - Searching by approvers (not to be confused with approved_by) in FOSS isn't supported.

This will enable the frontend team to add an "approvers" entry to the getMergeRequests query search criteria.

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