feat: Implement compatibility with SD.Next

[FoxxMD]

Originally based on https://github.com/hdtv35/aiyabot but with my own modifications so both backends can work

• Adds a backend global var and constants for setting the backend type
• backend is determined by endpoint GET at startup
• auth and model names are backend-dependent

This PR is NOT ready yet. There are several SD.Next specific features in my own branch I want to pull in. But I wanted to get it out in the open in case anyone else using SD.Next wanted to give it a try and provide feedback.

[FoxxMD]

Everything is added:

• Full quality VAE draw option
• Gets loras from official endpoint (works for webui as well)

[Kilvoctu]

Looks like there are some conflicts. Could you resolve them, sorry it wouldn't have happened if I merged sooner 🙏

[FoxxMD]

Checking on an auth change...

[FoxxMD]

Ok, good to go.