Pass a callback to get clientToken

[scottrblock]

Hi there, thanks for the work on this project.

What I'd like to do is pass a function to createYjsProvider to get the clientToken, instead of passing it as a string.

This allows the provider to be initially created and passed to wherever it needs to go without having to wait for an http request for the token until later.

This is similar to how y-partykit works - https://github.com/partykit/partykit/blob/main/packages/y-partykit/src/provider.ts#L568-L580

This looks like a pretty straightforward change, happy to take an initial stab at it, but figured I'd post it here first.