Add support for FLUX ControlNet models (XLabs and InstantX)

[RyanJDick]

Summary

Add support for FLUX ControlNet models (XLabs and InstantX).

QA Instructions

• [ ] SD1 and SDXL ControlNets, since the ModelLoaderRegistry calls were changed.
• [x] Single Xlabs controlnet
• [x] Single InstantX union controlnet
• [x] Single InstantX controlnet
• [ ] Single Shakker Labs Union controlnet
• [x] Multiple controlnets
• [x] Weight, start, end params all work as expected
• [x] Can be used with image-to-image and inpainting.
• [x] Clear error message if no VAE is passed when using InstantX controlnet.
• [x] Install InstantX ControlNet in diffusers format from HF repo (InstantX/FLUX.1-dev-Controlnet-Union)
• [ ] Test all FLUX ControlNet starter models

Merge Plan

No special instructions.

Checklist

• [x] The PR has a short but descriptive title, suitable for a changelog
• [x] Tests added / updated (if applicable)
• [ ] Documentation added / updated (if applicable)