Support `editor_register_get_classes_used_callback` with `api-custom-json` feature

[Yarwin]

godotengine/godot#104129 added editor_register_get_classes_used_callback which can be used to limit classes included in particular project – we should provide a list of used classes in case if api-custom-json feature is used.

Without api-custom-json || lazy-function-tables all the classes are required anyway – thus supporting this callback in other cases makes no sense.

As of today figuring out what classes are being used by given project is not being done by godot-rust and we shouldn't provide this functionality; It is user responsibility to provide proper extension_api.json from which used classes can be derived (in the future someone might create command line tool or plugin to support that).

[Bromeon]

In the past, I was against an environment variable to filter classes, but after some thought I believe there is some value in it:

1. The logic to filter classes based on extension_api.json is not completely trivial, needs quite a bit of jq shenanigans. Most notably, it needs users to delve into the JSON structure in the first place.
2. We can additionally specify classes that godot-rust needs internally (e.g. Engine, ClassDB, ...).
3. We can also recursively add all base classes of specified classes, which can be rather tedious for users.

So, an option is: for api-custom and api-custom-json, look for an environment variable like:

GODOT4_EXPOSED_CLASSES=Node2D,Area2D,Sprite2D,...

This would be all Godot names, comma-separated. We could then only provide codegen for these classes + their base classes + essential classes.

I'm not quite sure if we should do this without api-custom*, because users might expect the same crate API and not subtly let it influence by environment variables? Especially annoying when the env var is floating around somewhere globally.