Add support for cooperative cancellation

[AvenDonn]

Hello. I've found this package in the process of replacing a different older package. The older package didn't offer any async/await anyway, so this one is already an improvement.

I've noticed that methods I would normally expect to support cancellation just don't.

I'd like to be able to pass a cancellation token of my own to Start, StartOrFail, Stop, StopOrFail.

I see you already internally manage a cancellation token and pass it on to the real underlying method calls to the websocket client. Could you provide overloads that allow me to pass my own token and link to it?

I'm not quite sure I've implemented the cancellation in the way you would have liked, so please use my PR as a suggestion. (I'm not sure how you'd like to combine with the cancellation token source you save at the class instance level)

There are considerations. Should then token passed into Start be able to also cancel the whole Listen process after Start has finished? Arguably not. Feel free to modify my code as you see fit.