This adds two functions to `GDExtensionClassCreationInfo` that allow for developers to supply a generic virtual call function along with user data to be sent to that call.
If `get_virutal_call_data_func` is not null, extensions call this function to get user data to pass to a supplied `call_virtual_with_data_func`. Both must be provided is one is provided.
If `get_virtual_call_data_func` is null, Godot falls back to the old `get_virtual_func` logic.
Fixes#63275
Co-authored-by: David Snopek <dsnopek@gmail.com>