Adding backend support for summarizing a new merge request

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/142739



Merged-by: Madelein van Niekerk <mvanniekerk@gitlab.com>
Approved-by: Madelein van Niekerk <mvanniekerk@gitlab.com>
Approved-by: mo khan <mo@mokhan.ca>
Reviewed-by: Gary Holtz <gholtz@gitlab.com>
Reviewed-by: Patrick Bajao <ebajao@gitlab.com>
Reviewed-by: Madelein van Niekerk <mvanniekerk@gitlab.com>
Co-authored-by: Gary Holtz <gholtz@gitlab.com>