What does this MR do and why?

This adds several backend filters for the upcoming Vue merge request list:

* :approver, filter by possible approver of a merge request

• :approved_by, filter by someone who approved a merge request
• :my_reaction_emoji, filter by emoji you've applied
• :merged_by, filter by merger of a merge request
• :release, filter by release tag

The frontend doesn't use these (yet), but for extra credit you could confirm by merging these MRs:

• Add My Reaction filter to Vue merge request lis... (!159807 - merged)
• Add merged-by filter to Vue merge request list app (!159808 - merged)
• Add approved-by filter to Vue merge request lis... (!160005 - merged)

Then enable the vue_merge_request_list feature flag and check the merge requests search bar of a project.

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.

Related to:

• Add my-reaction filter to Vue merge request lis... (#442920 - closed)
• Add release filter to Vue merge request list app (#442919 - closed)
• Add approved-by filter to Vue merge request lis... (#442916 - closed)
• Add approver filter to Vue merge request list app (#442915 - closed)
• Add merged-by filter to Vue merge request list app (#442914 - closed)