hold cancel session (#6750)

## Problem

In a recent refactor, we accidentally dropped the cancel session early

## Summary of changes

Hold the cancel session during proxy passthrough

proxy/src/proxy.rs

@@ -331,6 +331,7 @@ pub async fn handle_client<S: AsyncRead + AsyncWrite + Unpin>(
         compute: node,
         req: _request_gauge,
         conn: _client_gauge,
+        cancel: session,
     }))
 }
 

proxy/src/proxy/passthrough.rs

@@ -1,4 +1,5 @@
 use crate::{
+    cancellation,
     compute::PostgresConnection,
     console::messages::MetricsAuxInfo,
     metrics::NUM_BYTES_PROXIED_COUNTER,
@@ -57,6 +58,7 @@ pub struct ProxyPassthrough<S> {
 
     pub req: IntCounterPairGuard,
     pub conn: IntCounterPairGuard,
+    pub cancel: cancellation::Session,
 }
 
 impl<S: AsyncRead + AsyncWrite + Unpin> ProxyPassthrough<S> {