Add merged-by filter to Vue merge request list app

What does this MR do and why?

For #442914 (closed)

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

Merged-By can only be inclusive (=).

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 Merged-By	Screencast_from_2024-09-03_18-03-31

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. Merge an MR (this is annoying, I know)
3. Navigate to the Merge Request list e.g. http://gdk.test:3000/[YOUR USERNAME]/[THE PROJECT NAMESPACE]/-/merge_requests
4. Begin typing merged-by in the search bar and/or select the Merged-By filter
5. Choose = and 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