In particular this:

* Adds an enum to the project_settings model with squash defaults
* Adds logic to reject merges when they don't meet the criteria
* Adds specs for the above
* Adds some endpoints for the frontend work
* Adds specs for the above

Thanks to @enzuru for the awesome groundwork on this! A lot of the code
is taken from his initial MR:

https://gitlab.com/gitlab-org/git lab/-/merge_requests/27833