Alternative async runtimes

[xDarksome]

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.