Backend for advanced search merge requests author facet

Refer to &9937 (closed) for backend and frontend naming for filter to keep consistent with filtering consolidation

1. Pass down author_username param to MergeRequestClassProxy
2. Add a feature flag
3. Update MergeRequestClassProxy behind the FF to:

if author_username: user = find user by author_username; add filter for author_id = user.id
if not[author_username]: user = find user by not_author_username; add must_not for author_id = user.id