Support for Forward Auth

## Description:

I was very impressed by Envoy Gateway at kubecon last week, and is looking into if we can migrate to it from ingress-nginx!

We are heavy users of oauth2proxy running as a service, by utilizing ingress-nginx forward auth.

It works by forwarding all requests to oauth2proxy, and if it responds with a 2xx the request is authorized and can continue to its original destination.

If the user is not authorized it will respond with a 302 that will redirect the user to a login endpoint (in our case Azure Entra ID), and after completion redirect to oauth2proxy, that will store a cookie and redirect the user back to the original target endpoint.

It is very important that all responses from oauth2proxy that is not a 2xx response should be sent to the user.

## Relevant links:
Similar to this, https://github.com/envoyproxy/gateway/issues/4562, but it seems they are running as Oauth2proxy as a proxy while we are using it as a `middleware`

### oauth2proxy

https://oauth2-proxy.github.io/oauth2-proxy/

### Traefik
This is also how Traefik handles ForwardAuth - https://doc.traefik.io/traefik/middlewares/http/forwardauth/