Add Latent-Interposer for SD15 and SDXL upscale models mixing

[DavideAlidosi]

First, congratulations on the work you have done on WebUI.
I would like to ask if it would be possible to integrate in the upscale options the Latent-Interposer system, already implemented on ComfyUI, which allows to combine the latent spaces of SD15 and SDXL.
The idea would be to take advantage of the increased capabilities of SDXL and then perform a refiner upscale with an SD15 model.
In the past few days I have already contacted city96, the author of the original node for Comfyui (city96/SD-Latent-Interposer#5), but I realized that I do not know enough about A1111 and Python to do the work myself.
Since you have already taken care of various aspects pertaining to this kind of functionality for A1111, I hope you will find it as interesting as I do.
Thank you.