Introduce AI-powered settings UI behind FF

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/138337



Merged-by: Nikola Milojevic <nmilojevic@gitlab.com>
Approved-by: Paulina Sedlak-Jakubowska <psedlak-jakubowska@gitlab.com>
Approved-by: Nikola Milojevic <nmilojevic@gitlab.com>
Approved-by: Simon Knox <simon@gitlab.com>
Reviewed-by: Simon Knox <simon@gitlab.com>
Co-authored-by: Aleksei Lipniagov <alipniagov@gitlab.com>