GD-78: Add Support for .Net GDExtension point

[MikeSchulze]

Is your feature request related to a problem? Please describe.
With Godot.Bindings we are able to provide gdunit4.api as a Godot extension point.

Describe the solution you'd like
Use the new project godot-dotnet to build an GDExtension.
This will allow us to use the api direct in Godot.

Describe alternatives you've considered
The alternative is actually to load the code as CSharpScript and build an GDScript bridge to access the C# API.

Additional context
Add any other context or screenshots about the feature request here.

[GeorgeS2019]

@MikeSchulze

It is now possible to use libgodot.dll for > Godot 4.3.x for windows using c#.
migeran/libgodot_project#1 (comment)

This would mean this feature request is needed to improve the reliability of using c# bindings for GDExtension
JiepengTan/libgodot_llgo_demo#4 (comment)

We encounter the challenges that the existing godot-dotnet is not up-to-date for the latest godot > 4.3.x

We many need community effort to have a good test coverage of the c# bindings exposed by the GDExtension