Can you add some clarity around this?

Depending on rust-extensions while trying to maintain support for windows and for async (whenever possible) is challenging with the current setup, as containerd-shim fails to build on windows when the async feature is enabled.
Making the feature a no-op on windows would work around this issue, allowing the use of async on unix platforms, and sync on windows.

That being said, I am currently collaborating with ttrpc-rust to add async support for windows there.
Once that's merged I would like to give it a go to adding async support for windows here.
I think overall that would be better than what's proposed in this PR.