gio/win32: add GMemoryMonitorWin32

Windows has CreateMemoryResourceNotification() API:

https://docs.microsoft.com/en-us/windows/win32/api/memoryapi/nf-memoryapi-creatememoryresourcenotification

It only notifies whether "Available physical memory is running low."

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>