Improve support for AdditionalAppExtensions

There are numerous issues around our AdditionalAppExtensions support:

* [ ] https://github.com/xamarin/xamarin-macios/issues/19243
* [ ] https://github.com/xamarin/xamarin-macios/issues/19432
* [x] https://github.com/xamarin/xamarin-macios/issues/19242
* [x] https://github.com/dotnet/macios/issues/23424

Also people have trouble using it, see:

* https://github.com/xamarin/xamarin-macios/issues/15315
* https://github.com/dotnet/maui/issues/19104

So in addition to fixing the above-mentioned issues, we should also:

* [ ] Write more documentation.
* See: https://learn.microsoft.com/en-us/xamarin/ios/platform/ios14/
* See: https://github.com/chamons/xamarin-ios-swift-extension/blob/master/App/net6/TestApplication/TestApplication.csproj
* [ ] Write more tests.
* [ ] Include a test building remotely from Windows as well.