Remote Header authentication

[jvoss]

Added Remote Header authentication configuration and tested with Caddy:

caddy_config: |
  :8080 {
    route /static* {
      uri strip_prefix /static
      root * {{ nautobot_root }}/static
      file_server
    }

    reverse_proxy http://127.0.0.1:8001 {
      header_up Remote-User test
    }
  }

This change also allows multiple authentication methods to be configured simultaneously.

Resolves #14