Add `onConnectionEstablished` callback to `Pond`

In Pond connection options, we can register an onConnectionLost handler that is triggered when the application cannot communicate with Actyx over the websocket connection.

If we also had an onConnectionEstablished handler, those two could be used to easily provide visual feedback in user facing applications.

Props to @CCFrankUrbainsky for bringing this up. Frank, do you have anything to add regarding your requirements?