Add async spawn support

[kennykerr]

Building on #3221 and #3213, this update concludes support for async implementations. You can now both consume and produce the four async interfaces defined by the Windows Runtime directly in Rust.

• IAsyncAction
• IAsyncOperation<T>
• IAsyncActionWithProgress<P>
• IAsyncOperationWithProgress<T, P>

As illustrated in #3221, if you happen to have a value ready to return, you can simply use the ready function as follows:

fn main() -> Result<()> {
    let ready = IAsyncOperation::ready(Ok(123));

    assert_eq!(ready.get()?, 123);

    Ok(())
}

If you need to spin up a background thread to wait for or calculate the value to return, you can use the spawn function as follows:

use windows::{core::*, Foundation::*};

fn main() -> Result<()> {
    let ready = IAsyncOperation::spawn(|| {
        std::thread::sleep(std::time::Duration::from_millis(1000)); // some lengthy operation
        Ok(123)
    });

    assert_eq!(ready.get()?, 123);

    Ok(())
}

While the ready implementations return immediately, the spawn implementations will take the closure and pass it by value to the Windows thread pool. This is far more efficient and scalable than creating a new thread each time spawn is called.

I have also included extensive tests for both ready and spawn implementations in this update.

[kennykerr]

Disabled the lib workflow as its proving unreliable.