Need sample for WinAppSDK / WinUI3 background tasks

[jonwis]

https://learn.microsoft.com/windows/uwp/launch-resume/create-and-register-a-winmain-background-task shows how to register a background task for Win32 apps. Please create a sample showing how to use this functionality from a WinAppSDK based app (with the AppInstance) and a WinUI3 app.