add placeholder for agent forwarding

It's not fully implemented by libssh2.
refs: libssh2/libssh2#535

wezterm-ssh/src/pty.rs

@@ -144,6 +144,19 @@ impl crate::session::SessionInner {
         let mut channel = sess.channel_session()?;
 
         channel.handle_extended_data(ssh2::ExtendedData::Merge)?;
+
+        /* libssh2 doesn't properly support agent forwarding
+         * at this time:
+         * <https://github.com/libssh2/libssh2/issues/535>
+        if let Some("yes") = self.config.get("forwardagent").map(|s| s.as_str()) {
+            log::info!("requesting agent forwarding");
+            if let Err(err) = channel.request_auth_agent_forwarding() {
+                log::error!("Failed to establish agent forwarding: {:#}", err);
+            }
+            log::info!("agent forwarding OK!");
+        }
+        */
+
         channel.request_pty(
             &newpty.term,
             None,