Update plugins demo to use custom Resources

[Calinou]

Which demo project is affected: https://github.com/godotengine/godot-demo-projects/tree/master/plugins

Description: The Plugins demo currently doesn't rely on custom Resources and uses Nodes instead. This was because of limitations in Godot 3.x that made it impossible to export custom Resources. However, this limitation has been lifted in Godot 4.0, so the demo should be updated to use the new recommended usage patterns.