Add an API setting for configurable squash defaults

Problem to solve

The issue for Configurable squash defaults has landed on gitlab.com, but some customers could still use access to the feature via the API.

Intended users

Users that interact with the API to make mass changes, for example: One user has a python script/terraform that bulk changes settings to all their projects.

Proposal

Add a setting in the API that would use the same options as #17613 (closed):

• Never (do not show the squash option, do not squash)
• Allowed (default, current behavior, show the squash option, unchecked but user can enable)
• Enabled (show the squash option, checked but user can disable)
• Always (show the squash option, check and the user cannot disable)

Further details

TBD?