Feature Request: Add support for authentication by reverse proxy

[max-tet]

Summary:
Add configuration options: 1) enable proxy auth, 2) proxy auth http-header (e.g. X-Authenticated-User).

If proxy auth is enabled, user login is bypassed and the user that is present in the specified header is used.

Optional: if no such user exists, automatically create one.

How important this is to me and why:
Some on-premise hosting approaches put everything behind a reverse proxy like Traefik and let authentication be handled there. See e.g. here. This feature would enable this kind of deployment.

Importance: High - because it might be really easy and therefore a quick win.

Use cases:

1. on-premise hosting with reverse-proxy auth
2. personal self-hosting

Additional context / similar features:
Example: Filebrowser

Thanks a lot, cheers!

[p-baum]

The broken link above should be https://doc.traefik.io/traefik/middlewares/http/forwardauth/

I would also be very pleased to see this implemented.