Add Variant::object_id()

[Bromeon]

Follow upstream godotengine/godot#97119.

Also improved doc + test coverage for the existing Callable::object_id() + Signal::object_id().

[GodotRust]

API docs are being generated and will be shortly available at: https://godot-rust.github.io/docs/gdext/pr-914

[Bromeon]

I kept this open, as the #[cfg(since_api = "4.4")] is inactive for the current doc tasks, because those run on default api-* feature (currently 4.3). We would need api-custom feature + nightly Godot to display latest symbols, but that's a bigger change. I made a corresponding comment in the website repo: godot-rust/godot-rust.github.io@2a83b54

So I'll merge this, but it won't be available in online docs until 4.4 is released. People who want 100% accurate docs with 4.4-dev features can build docs locally using api-custom.