Alternative async runtimes

Now that `tokio-quiche` has landed, are you open to making it more runtime agnostic?
After giving it a _quic_ look it seems to me that the majority of the code isn't dependent on the `tokio` runtime directly.

You've built a nice high level abstraction on top of `quiche` that can be useful outside of `tokio` ecosystem. Personally, I would like to see it being `async-quiche` with `tokio` being just a plugin.

I'm willing to put some work into it, if you're interested.