Add an API endpoint to download git repository snapshots

What does this MR do?

Implements a GET /api/v4/projects/:id/snapshot API endpoint. This allows an instance administrator to download a .tar file containing the raw git repository files.

Are there points in the code the reviewer needs to double check?

We depend on gitlab-workhorse and gitaly for the heavy lifting. Here, we just need to be sure that the API endpoint is in a good place, and the authentication can easily be overridden in EE.

Why was this MR needed?

~Geo %10.7 Deliverable

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by Backend
  • Has been reviewed by Security
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered
• End-to-end tests pass (package-and-qa manual pipeline job)

What are the relevant issue numbers?

Closes #39345 (closed)