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/csharp_script.cpp

@@ -2179,7 +2179,9 @@ bool CSharpScript::_update_exports(PlaceHolderScriptInstance *p_instance_to_upda
 	if (exports_invalidated)
 #endif
 	{
+#ifdef TOOLS_ENABLED
 		exports_invalidated = false;
+#endif
 
 		changed = true;
 
@@ -2222,6 +2224,7 @@ bool CSharpScript::_update_exports(PlaceHolderScriptInstance *p_instance_to_upda
 						}
 					});
 
+#ifdef TOOLS_ENABLED
 			GDMonoCache::managed_callbacks.ScriptManagerBridge_GetPropertyDefaultValues(this,
 					[](CSharpScript *p_script, GDMonoCache::godotsharp_property_def_val_pair *p_def_vals, int32_t p_count) {
 						for (int i = 0; i < p_count; i++) {
@@ -2233,6 +2236,7 @@ bool CSharpScript::_update_exports(PlaceHolderScriptInstance *p_instance_to_upda
 							p_script->exported_members_defval_cache[name] = value;
 						}
 					});
+#endif
 		}
 	}