Add `user` actor to attention requests feature flag
Problem to Solve
Currently attention requests is behind a global feature flag. This means that it's either on for everyone or on for no one. This makes it hard to gain more feedback on the feature, without impacting workstreams of GitLab and other companies who rely on GitLab for their code review process.
Additional Details
In multiple instances we've relied on the global nature of the feature flag because we're impacting so many parts of the GitLab application. Per project and per group toggles for the feature flag aren't suitable because pieces in navigation and more global notification items won't be accessible with those limited scopes. Per user toggles while potentially possible present an additional management challenge as you need to roll the feature flag out for every user individually.
Proposal
Update the feature flag to accept a user
actor so that the feature could be selectively enabled for users looking to test the feature.