Improve support for AdditionalAppExtensions

[rolfbjarne]

There are numerous issues around our AdditionalAppExtensions support:

• AdditionalAppExtensions: ITMS-90075: "This bundle is invalid. The application-identifier entitlement is missing #19243
• [macOS] dotnet publish fails when using AdditionalAppExtensions #19432
• AdditionalAppExtensions: .entitlements file location - better lookup or error #19242

Also people have trouble using it, see:

• [Prototype] Swift Code Extensions to Xamarin applications #15315
• Cannot build iOS widget apps with extensions (ditto exited with code 1) maui#19104

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

• Write more documentation, in particular targeting .NET.
  • 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.