Backend work for configurable project defaults for squashing merges

What does this MR do?

This adds a backend component to #17613 (closed) - @sming-gitlab will follow up with the frontend work here!

In particular this:

• Adds an enum to the projects 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

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done