feature/Allow async events add url mutation

[hathamneh]

This PR adds two features:

• allow modification url after the websocket is built. ex: ws.url = 'some_new_url'
• allow async events handlers: ex:

   WebsocketBuilder.onRetry(async (ws, ev) => {
     await someAsynTask();
   })

---

The use case needed those is:
We have a secured API that requires a temporary token per socket connection, so what is needed is to get a new token asynchronously then append it to the socket url:

   WebsocketBuilder.onRetry(async (ws, ev) => {
     const newToken = await someAsynTask();
     ws.url = `/myUrl?token=${newToken}`;
   })

[jjxxs]

Thanks for the PR. But I've decided to close this, reasoning see #15.