Expose non-template versions of vararg methods

### Godot version

4.3.stable

### godot-cpp version

4.3

### System information

MacOS 15.2

### Issue description

For custom scripting support and other dynamic uses, it would be nice to expose vararg methods as non-templated.

Currently, the dynamic (non-template) version (`_internal`) for `classes` and `utility_functions` already exists, although marked `private:` . As for `builtin_classes` there are no equivalent, the closes thing would be the _private_ `_method_bindings` struct but those are raw method pointers doesn't give us nice signature and return value handling.

If this suggestion is accepted, I can raise a PR to remove the private accessor for `classes` and `utility_functions`, and refactor `builtin_classes` bindings to expose the corresponding `*_internal` methods.

### Steps to reproduce

N/A

### Minimal reproduction project

N/A