proxy: Refactor http conn pool (#9785)

- Use the same ConnPoolEntry for http connection pool.
- Rename EndpointConnPool to the HttpConnPool.
- Narrow clone bound for client

Fixes #9284

proxy/src/serverless/sql_over_http.rs

@@ -779,6 +779,7 @@ async fn handle_auth_broker_inner(
     let _metrics = client.metrics();
 
     Ok(client
+        .inner
         .inner
         .send_request(req)
         .await