Add approved-by filter to Vue merge request list app

What does this MR do and why?

For #442916 (closed)

Adds the Approved-By filter to the Vue-implemented Merge Request list (behind the vue_merge_request_list feature flag).

Approved-By can be inclusive (=) or exclusive (!=).

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

Before	After
With the vue_merge_request_list feature flag enabled, there is no option to filter by Approved-By	Screencast_from_2024-08-29_12-45-01

How to set up and validate locally

1. Enable the vue_merge_request_list feature flag in the rails console rails c => Feature.enable( :vue_merge_request_list )
2. Approve an MR or two
3. Navigate to the Merge Request list e.g. http://gdk.test:3000/[YOUR USERNAME]/[THE PROJECT NAMESPACE]/-/merge_requests
4. Begin typing approved-by in the search bar and/or select the Approved-By filter
5. Choose = or !=, plus your username
6. Submit the search / filter
   • Try resubmitting the URL in a new tab to recall the same filters in the UI
   • Try clearing the filter bar and submit the (empty) search to see the URL parameters clear
   • Try the other filter option (= or !=) to see a different set of MRs appear

Bonus Validation

1. Sign out, sign in as a different user, and approve the same MR(s) that you approved with your first user
2. Go back to the MR list
3. Include BOTH users in your filter, e.g. (Approved-By=youruser, Approved-By=otheruser) and submit to find only the MRs with both approvals