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/serverless/sql_over_http.rs

@@ -189,7 +189,7 @@ fn get_conn_info(
     }
 
     let endpoint = endpoint_sni(hostname, &tls.common_names)?.context("malformed endpoint")?;
-    ctx.set_endpoint_id(Some(endpoint.clone()));
+    ctx.set_endpoint_id(endpoint.clone());
 
     let pairs = connection_url.query_pairs();