std.Io.Threaded: Darwin implementation of `netLookup`

[andrewrk]

Extracted from #25592.

The macOS implementation of netLookup should use CFHostStartInfoResolution / CFHostCancelInfoResolution to obtain these two properties:

• asynchronously add to results queue
• cancellation integration

    if (native_os.isDarwin()) {
        // TODO use CFHostStartInfoResolution / CFHostCancelInfoResolution
    }