fix(proxy): forward notifications from authentication (#9948)

Fixes https://github.com/neondatabase/cloud/issues/20973. 

This refactors `connect_raw` in order to return direct access to the
delayed notices.

I cannot find a way to test this with psycopg2 unfortunately, although
testing it with psql does return the expected results.

libs/proxy/postgres-protocol2/src/message/backend.rs

@@ -541,6 +541,10 @@ impl NoticeResponseBody {
     pub fn fields(&self) -> ErrorFields<'_> {
         ErrorFields { buf: &self.storage }
     }
+
+    pub fn as_bytes(&self) -> &[u8] {
+        &self.storage
+    }
 }
 
 pub struct NotificationResponseBody {