Merge branch '415584-assign-user-add-on-mutation' into 'master'

Add mutation userAddOnAssignmentCreate

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/126964



Merged-by: Abdul Wadood <awadood@gitlab.com>
Approved-by: Halil Coban <hcoban@gitlab.com>
Approved-by: Abdul Wadood <awadood@gitlab.com>
Reviewed-by: Abdul Wadood <awadood@gitlab.com>
Reviewed-by: Bishwa Hang Rai <bhrai@gitlab.com>
Reviewed-by: Halil Coban <hcoban@gitlab.com>
Co-authored-by: Bishwa Hang Rai <bhrai@gitlab.com>