Enhancement: Support proxy-based authentication

Rest-server now supports authentication via HTTP proxy headers. This feature can
be enabled by specifying the username header using the `--proxy-auth-username`
option (e.g., `--proxy-auth-username=X-Forwarded-User`).

When enabled, the server authenticates users based on the specified header and
disables Basic Auth. Note that proxy authentication is disabled when `--no-auth`
is set.

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