Add assignee filter to Vue merge request list app

What does this MR do and why?

For #442912 (closed)

Adds the MR assignee filter to the Vue MR list search bar.

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
Filter option dropdown
Filtered by assignee	x

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. Have or create an MR that has an assignee
   • It will help to have at least one other MR that does not have an assignee
3. Navigate to the Merge Request list e.g. http://gdk.test:3000/[YOUR USERNAME]/[THE PROJECT NAMESPACE]/-/merge_requests
4. Begin typing assignee in the search bar and select the assignee filter token
5. Select the user who is assigned to your MR in step 2
6. Submit the search / filter
   1. Try this with another user
   2. Try refreshing the page completely to test the URL search parameter on initial load
   3. Try clearing the assignee token filter and submitting that to see all the MRs become visible

Caveats

Please note there are 2.5 known issues:

1. The current back end implementation does not support the "negated" (!=) filter, so that's simply not implemented here - which does not match with the non-Vue list
2. The current back end implementation does not support wildcard (None, Any) filters
   1. They still display in the FE filter token, but don't actually modify the filter results