Add label filter to Vue merge request list app

What does this MR do and why?

For #442918 (closed)

Adds the label 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 label	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 a label
   • It will help to have one other MR that does not have a label
3. Navigate to the Merge Request list e.g. http://gdk.test:3000/[YOUR USERNAME]/[THE PROJECT NAMESPACE]/-/merge_requests
4. Begin typing label in the search bar and select the label filter token
5. Select the label that is assigned to your MR in step 2
6. Submit the search / filter
   1. Try this with another label
   2. Try refreshing the page completely to test the URL search parameter on initial load
   3. Try clearing the label token filter and submitting that to see all the MRs become visible
   4. Try this with the Any/None wildcard options.
   5. Try with more than one label, which should further reduce the results

Caveats

1. While the back end for this query supports negated labels, we still need the authoritative list of negatable filters. Because of that missing value, negated filters will not actually filter the results until a future iteration.