feat: stabilize unique_ptr based initialize function

[jackgerrits]

I am essentially just stabilizing the initialize_experimental function which has been working well for quite some time. With the changes of adding skip_model_load and removing the custom stack functionality from the stable interface. The custom stack is still available from initialize_experimental

std::unique_ptr<VW::workspace> initialize(std::unique_ptr<config::options_i> options,
    std::unique_ptr<VW::io::reader> model_override_reader = nullptr, bool skip_model_load = false,
    driver_output_func_t driver_output_func = nullptr, void* driver_output_func_context = nullptr,
    VW::io::logger* custom_logger = nullptr);