Add support for FreeU. See:
- https://huggingface.co/docs/diffusers/main/en/using-diffusers/freeu
- https://github.com/ChenyangSi/FreeU
Implementation:
- `ModelPatcher.apply_freeu()` handles the enabling freeu (which is very simple with diffusers).
- `FreeUConfig` model added to hold the hyperparameters.
- `freeu_config` added as optional sub-field on `UNetField`.
- `FreeUInvocation` added, works like LoRA - chain it to add the FreeU config to the UNet
- No support for model-dependent presets, this will be a future workflow editor enhancement
Closes#4845
