Support registering main loop callback in Godot 4.5

[beicause]

Related to:

• [GDExtension] Add function to register main loop callbacks godotengine/godot#106030

Related godot-cpp update: godotengine/godot-cpp#1792
The startup callback is useful for registering user's singleton that accesses server singletons such as RenderingServer.

[beicause]

Godot 4.5.stable has released. I'm interested in implementing this (probably add them in ExtensionLibrary trait), but it seems I have to wait for gdext to add api-4.5.

[Bromeon]

Yes, to merge, you'll have to wait until we release v0.4. However, you can already start working on it with api-custom.