Add an API endpoint to download git repository snapshots (EE)

What does this MR do?

EE version of https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/18173

In addition to the CE functionality, this MR adds the ability for a Geo secondary to get a snapshot of a repository from a Geo primary. It does not currently attach this functionality to anything.

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

In the API, we end up needing to punch a hole through the user_project access checks, which is disconcerting. I've put a fix in, but I don't think it's the right fix.

Why was this MR needed?

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
• If paid feature, have we considered GitLab.com plan and how it works for groups and is there a design for promoting it to users who aren't on the correct plan
• End-to-end tests pass (package-qa manual pipeline job)

What are the relevant issue numbers?

Related to https://gitlab.com/gitlab-org/gitlab-ce/issues/39345

Closes https://gitlab.com/gitlab-org/gitlab-ee/issues/4767