Implement two-phase timeout in proxy_handler:
- Inner timeout uses per-site upstream_connect_timeout_secs (default 5s)
for the connect + first-byte phase
- Outer timeout uses upstream_request_timeout_secs (default 60s) for the
full request/response cycle
- Set connect_timeout on HttpConnector for both HTTP and HTTPS clients
(default 5s) to enforce TCP-level connect timeouts
- Use wrap_connector for HTTPS client to apply connect_timeout on the
underlying HttpConnector
- Add Ok(Err(_)) handler for connect timeout returning 504 Gateway Timeout