C#: Initial NativeAOT support

This commit adds initial support for games exported as NativeAOT shared
libraries.

At this moment, the NativeAOT runtime is experimental. Additionally,
Godot is not trim-safe as it still makes some use of reflection.
For the time being, a rd.xml file is needed to prevent code triming:

```
<Directives xmlns="http://schemas.microsoft.com/netfx/2013/01/metadata">
  <Application>
    <Assembly Name="GodotSharp" Dynamic="Required All" />
    <Assembly Name="GAME_ASSEMBLY" Dynamic="Required All" />
  </Application>
</Directives>
```

These are the csproj changes for publishing:

```
  <PropertyGroup>
    <NativeLib>Shared</NativeLib>
  </PropertyGroup>
  <ItemGroup>
    <RdXmlFile Include="rd.xml" />
    <PackageReference Include="Microsoft.DotNet.ILCompiler" Version="7.0.0-*" />
  </ItemGroup>
```

More info:
- https://github.com/dotnet/runtimelab/blob/feature/NativeAOT/docs/using-nativeaot/compiling.md
- https://github.com/dotnet/runtimelab/tree/feature/NativeAOT/samples/NativeLibrary
- https://github.com/dotnet/runtimelab/blob/feature/NativeAOT/docs/using-nativeaot/rd-xml-format.md

modules/mono/editor/Godot.NET.Sdk/Godot.SourceGenerators/GodotPluginsInitializerGenerator.cs

@@ -26,14 +26,16 @@ namespace GodotPlugins.Game
 {
     internal static partial class Main
     {
-        [UnmanagedCallersOnly]
-        private static godot_bool InitializeFromGameProject(IntPtr outManagedCallbacks)
+        [UnmanagedCallersOnly(EntryPoint = ""godotsharp_game_main_init"")]
+        private static godot_bool InitializeFromGameProject(IntPtr godotDllHandle, IntPtr outManagedCallbacks)
         {
             try
             {
+                DllImportResolver dllImportResolver = new GodotDllImportResolver(godotDllHandle).OnResolveDllImport;
+
                 var coreApiAssembly = typeof(Godot.Object).Assembly;
 
-                NativeLibrary.SetDllImportResolver(coreApiAssembly, GodotDllImportResolver.OnResolveDllImport);
+                NativeLibrary.SetDllImportResolver(coreApiAssembly, dllImportResolver);
 
                 ManagedCallbacks.Create(outManagedCallbacks);