Merge branch '432212/mhamda/assign_sm_users' into 'master'

Refactor UserAddOnAssignments and the GQL API

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/138157



Merged-by: Bob Van Landuyt <bob@gitlab.com>
Approved-by: Bishwa Hang Rai <bhrai@gitlab.com>
Reviewed-by: Bob Van Landuyt <bob@gitlab.com>
Reviewed-by: Bishwa Hang Rai <bhrai@gitlab.com>
Reviewed-by: Mohamed Hamda <mhamda@gitlab.com>
Co-authored-by: Mohamed Hamda <mhamda@gitlab.com>