Support for Auth proxy

### Prerequisites

- [x] I am running the latest version
- [x] I checked the documentation and found no answer
- [x] I checked to make sure that this issue has not already been filed

### Problem Description

I'm running pomerium.io as the reverse proxy to Adguard Home's web UI. It would be great if Adguard Home web UI can support identity-aware proxy like Pomerium.

### Proposed Solution
When the incoming HTTP request contains an `X-Foo-Bar: username` header, Adguard Home web UI should treat it as an authenticated HTTP request as if `username` is logged in.

The header `X-Foo-Bar` should be configurable.

(See the doc on Grafana's auth proxy support: https://grafana.com/docs/grafana/latest/auth/auth-proxy/)

### Alternatives Considered

An option to disable authentication completely, and relying on the reverse proxy to authenticate the access to the web UI.

### Additional Information