Introduce extra_folder_paths feature to allow users to add additional
model roots that are managed by LoRA Manager but not shared with ComfyUI.
Changes:
- Add extra_folder_paths support in SettingsManager (stored per library)
- Add extra path attributes in Config class (extra_loras_roots, etc.)
- Merge folder_paths with extra_folder_paths when applying library settings
- Update LoraScanner, CheckpointScanner, EmbeddingScanner to include
extra paths in their model roots
- Add comprehensive tests for the new functionality
This enables users to manage models from additional directories without
modifying ComfyUI's model folder configuration.
