Add proxy_download to perform proxied sending of all files

What does this MR do?

Implements proxying all access to files stored remotely through Workhorse. The default is to proxy by default, as this is the best secure option. This is configurable for the administrator if needed.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Review
  • Has been reviewed by Backend
• 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-qa manual pipeline job)

What are the relevant issue numbers?

Closes https://gitlab.com/gitlab-org/gitlab-ee/issues/4703