Add MAUI example app to csharp samples

[weslleymurdock]

This covers #4077
I've created the sample application for both iOS & Android platforms, and some modifications on makefile to generate c# PINVOKE classes to the right project as well

[CLAassistant]

All committers have signed the CLA.

[trengginas]

I am stuck with these error when trying to build the app:

/usr/local/share/dotnet/sdk/8.0.403/Microsoft.Common.CurrentVersion.targets(2303,5): error MSB4803: The task "ResolveNativeReference" is not supported on the .NET Core version of MSBuild. Please use the .NET Framework version of MSBuild. See https://aka.ms/msbuild/MSB4803 for further details. [/pjsip/test/pjsip-apps/src/swig/csharp/pjsua2maui/pjsua2maui/pjsua2maui.csproj::TargetFramework=net8.0-android]
/pjsip/test/pjsip-apps/src/swig/csharp/pjsua2maui/pjsua2maui/Platforms/iOS/CallPageHandler.cs(4,40): error CS0234: The type or namespace name 'Compatibility' does not exist in the namespace 'Microsoft.Maui.Controls.Handlers' (are you missing an assembly reference?) [/pjsip/test/pjsip-apps/src/swig/csharp/pjsua2maui/pjsua2maui/pjsua2maui.csproj::TargetFramework=net8.0-ios]
/pjsip/test/pjsip-apps/src/swig/csharp/pjsua2maui/pjsua2maui/Platforms/iOS/AppDelegate.cs(6,28): error CS0246: The type or namespace name 'MauiUIApplicationDelegate' could not be found (are you missing a using directive or an assembly reference?) [/pjsip/test/pjsip-apps/src/swig/csharp/pjsua2maui/pjsua2maui/pjsua2maui.csproj::TargetFramework=net8.0-ios]
/pjsip/test/pjsip-apps/src/swig/csharp/pjsua2maui/pjsua2maui/Platforms/iOS/CallPageHandler.cs(16,33): error CS0246: The type or namespace name 'VisualElementRenderer<>' could not be found (are you missing a using directive or an assembly reference?) [/pjsip/test/pjsip-apps/src/swig/csharp/pjsua2maui/pjsua2maui/pjsua2maui.csproj::TargetFramework=net8.0-ios]

[weslleymurdock]

Hello
Which version of dotnet you`re using
Please post the dotnet setup details

[weslleymurdock]

Hi again
I'll try to create a global.json to configure the .ned sdk version as the same i've used