Allow access to raw files using HTTP Basic Auth

A customer requests the possibility to access raw files using HTTP-Basic Auth.

For some applications using a header, or a private token in the querystring, is not an option. Tools provide a basic login window that allows users to authenticate.

We could implement this as a part of https://gitlab.com/gitlab-org/gitlab-ce/issues/36203 where we could allow basic auth on some endpoints, minimizing the security implications.

This was brought up in https://gitlab.zendesk.com/agent/tickets/81253 (Internal only)