Compilation fails on windows

[waynr]

I have a user who is trying to build kubit from source, which pulls in kubert as a dependency. The user OS is Windows. They run into an issue with respect to signal handling in the shutdown module of this crate because it unconditionally uses tokio::signal::unix

It looks like there is a tokio::signal::windows module available that could be conditionally compiled based on platform feature flags, but the logic around signal handling will probably have to be different for each since it looks like the two platforms provide different approaches to signal handling.