Enhancement: Add support for proxy-based authentication

The server now supports authentication via a proxy header specified with the --proxy-auth flag (e.g., --proxy-auth=X-Forwarded-User).
When this flag is set, the server will authenticate users based on the given header and disable BasicAuth.
Note that --proxy-auth is ignored if --no-auth is set, as --no-auth disables all authentication.

https://github.com/restic/rest-server/issues/174
https://github.com/restic/rest-server/pull/307