Allow bulk reassignment by API endpoint

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/183690



Merged-by: Carla Drago <cdrago@gitlab.com>
Approved-by: Rez <rmarandi@gitlab.com>
Approved-by: Ashraf Khamis <akhamis@gitlab.com>
Approved-by: Alessio Caiazza <code.git@caiazza.info>
Approved-by: Carla Drago <cdrago@gitlab.com>
Reviewed-by: Ashraf Khamis <akhamis@gitlab.com>
Co-authored-by: James Nutt <jnutt@gitlab.com>