[Feature]: Securely sending data with `useSync`
### What's the feature?

I was previously using [Hocuspocus](https://github.com/ueberdosis/hocuspocus) for collaboration. I now want to use tldraw/sync.

However, I had implemented authentication logic via Hocuspocus - it was basically sending over a the token through the websocket at the initial message.

Ideally `useSync` would allow sending some arbitrary data securely which can be processed before the websocket is used.


### Where would you like this feature?

The developer package

### Contact Details

neosh11@gmail.com

### Code of Conduct

- [X] I agree to follow this project's Code of Conduct