Add main loop callbacks to ExtensionLibrary

[beicause]

Closes #1299.
Adds on_main_loop_startup, on_main_loop_frame, on_main_loop_shutdown callbacks to ExtensionLibrary for Godot 4.5.

This PR needs rebase once we have api-4.5 feature.

[GodotRust]

API docs are being generated and will be shortly available at: https://godot-rust.github.io/docs/gdext/pr-1313

[Bromeon]

Thanks for the update!

[Bromeon]

Thank you! Some thoughts:

• Godot uses "startup" and "shutdown", I think we should keep things consistent with the other godot-rust APIs and use "init" and "deinit" instead. Then we'd have on_level_init + on_main_loop_init, etc.
• Is there a strict order between the levels and the main loop? If yes, should we (not in this PR, but a follow-up) consider representing MainLoop as its own InitLevel?

[beicause]

• Godot uses "startup" and "shutdown", I think we should keep things consistent with the other godot-rust APIs and use "init" and "deinit" instead. Then we'd have on_level_init + on_main_loop_init, etc.
• Is there a strict order between the levels and the main loop? If yes, should we (not in this PR, but a follow-up) consider representing MainLoop as its own InitLevel?

on_main_loop_init feels off, the startup is actually after the MainLoop/SceneTree is created. It is the final step of Main::start() and after all init levels (shutdown is the vice versa, before all init levels).

I think it's better to make them separate functions, keeping on_init_level lean so you're less likely to make mistakes in it.