proxy: refactor auth backends (#9271)

preliminary for #9270 

The auth::Backend didn't need to be in the mega ProxyConfig object, so I
split it off and passed it manually in the few places it was necessary.

I've also refined some of the uses of config I saw while doing this
small refactor.

I've also followed the trend and make the console redirect backend it's
own struct, same as LocalBackend and ControlPlaneBackend.

proxy/src/serverless/websocket.rs

@@ -129,6 +129,7 @@ impl<S: AsyncRead + AsyncWrite + Unpin> AsyncBufRead for WebSocketRw<S> {
 
 pub(crate) async fn serve_websocket(
     config: &'static ProxyConfig,
+    auth_backend: &'static crate::auth::Backend<'static, (), ()>,
     ctx: RequestMonitoring,
     websocket: OnUpgrade,
     cancellation_handler: Arc<CancellationHandlerMain>,
@@ -145,6 +146,7 @@ pub(crate) async fn serve_websocket(
 
     let res = Box::pin(handle_client(
         config,
+        auth_backend,
         &ctx,
         cancellation_handler,
         WebSocketRw::new(websocket),