Add QA Feature Flag usage Cop

Closes gitlab-org/quality/quality-engineering/team-tasks#2759

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/154843



Merged-by: Peter Leitzen <pleitzen@gitlab.com>
Approved-by: Serhii Yarynovskyi <syarynovskyi@gitlab.com>
Approved-by: Chloe Liu <yliu@gitlab.com>
Approved-by: Peter Leitzen <pleitzen@gitlab.com>
Reviewed-by: Dan Davison <ddavison@gitlab.com>
Reviewed-by: Peter Leitzen <pleitzen@gitlab.com>
Co-authored-by: ddavison <ddavison@gitlab.com>