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/mod.rs

@@ -55,6 +55,7 @@ pub(crate) const SERVERLESS_DRIVER_SNI: &str = "api";
 
 pub async fn task_main(
     config: &'static ProxyConfig,
+    auth_backend: &'static crate::auth::Backend<'static, (), ()>,
     ws_listener: TcpListener,
     cancellation_token: CancellationToken,
     cancellation_handler: Arc<CancellationHandlerMain>,
@@ -110,6 +111,7 @@ pub async fn task_main(
         local_pool,
         pool: Arc::clone(&conn_pool),
         config,
+        auth_backend,
         endpoint_rate_limiter: Arc::clone(&endpoint_rate_limiter),
     });
     let tls_acceptor: Arc<dyn MaybeTlsAcceptor> = match config.tls_config.as_ref() {
@@ -397,6 +399,7 @@ async fn request_handler(
             async move {
                 if let Err(e) = websocket::serve_websocket(
                     config,
+                    backend.auth_backend,
                     ctx,
                     websocket,
                     cancellation_handler,