Support for integrated SDXL base + refiner sampler

[Fooocus](https://github.com/lllyasviel/Fooocus) by [lllyasviel](https://github.com/lllyasviel) introduced new type of sampler for SDXL, where both base and refiner model work together during single sampling pass - which allows refiner to use momentum from base, and improves quality of the final image (described with more details on Fooocus project page).

Fooocus looks like interesting project with nice optimizations, but it aims for simplicity - and hides most configuration options from the users. Implementation of this new sampler looks largely compatible with ComfyUI code, as it uses Comfy code. KSamplerWithRefiner itself (defined in [samplers_advanced.py](https://github.com/lllyasviel/Fooocus/blob/main/modules/samplers_advanced.py)) looks pretty similar to KSampler defined in [samplers.py](https://github.com/comfyanonymous/ComfyUI/blob/master/comfy/samplers.py). It would be nice to have it ported to ComfyUI, and available as integrated SDXL base + refiner sampler node, so people who don't mind playing with nodes could benefit from this optimization, too.