flatten proxy flow (#6447)

## Problem

Taking my ideas from https://github.com/neondatabase/neon/pull/6283 and
doing a bit less radical changes. smaller commits.

Proxy flow was quite deeply nested, which makes adding more interesting
error handling quite tricky.

## Summary of changes

I recommend reviewing commit by commit.

1. move handshake logic into a separate file
2. move passthrough logic into a separate file
3. no longer accept a closure in CancelMap session logic
4. Remove connect_to_db, copy logic into handle_client
5. flatten auth_and_wake_compute in authenticate
6. record info for link auth

proxy/src/bin/pg_sni_router.rs

@@ -272,5 +272,5 @@ async fn handle_client(
     let client = tokio::net::TcpStream::connect(destination).await?;
 
     let metrics_aux: MetricsAuxInfo = Default::default();
-    proxy::proxy::proxy_pass(ctx, tls_stream, client, metrics_aux).await
+    proxy::proxy::passthrough::proxy_pass(ctx, tls_stream, client, metrics_aux).await
 }