Support for Custom Servers/Auto-Load Singletons

### Godot version

4.1.1stable

### godot-cpp version

4.1.1stable

### System information

Arch, btw

### Issue description

I am trying to add a new Server to act as a something like an AutoLoad Singleton (a global resource that maintains state across the lifetime of the game application, and is given work by Nodes) in GDExtension. I run into the following issue however:
`extension/src/register_types.cpp:37:34: error: 'class godot::Engine' has no member named 'add_singleton'; did you mean 'get_singleton'?
37 | Engine::get_singleton()->add_singleton(Engine::Singleton("SteamAudioServer", SteamAudioServer::get_singleton()));
| ^~~~~~~~~~~~~
| get_singleton
extension/src/register_types.cpp:37:56: error: 'Singleton' is not a member of 'godot::Engine'
37 | Engine::get_singleton()->add_singleton(Engine::Singleton("SteamAudioServer", SteamAudioServer::get_singleton()));
`
It seems the add_singleton method is not exposed by GDExtension for the Engine singleton. A version of this code works fine with old-fashioned Modules, however. For this implementation the Server (or Singleton) needs to load before nodes in the load order. Any suggestions would be great.

### Steps to reproduce

Try to adapt a Custom Server module to GDExtension:
https://docs.godotengine.org/en/stable/contributing/development/core_and_modules/custom_godot_servers.html

### Minimal reproduction project

N/A