Add holding directory for IPAdapters

[fofr]

The most popular IPAdapter custom node https://github.com/cubiq/ComfyUI_IPAdapter_plus puts these models into the models/ipadapter directory.

This PR adds that holding directory as a default.

cc @cubiq

[cubiq]

Ideally you'd also add folder_names_and_paths["ipadapter"] = ([os.path.join(models_dir, "ipadapter")], supported_pt_extensions) to folder_paths.py

[fofr]

@cubiq Added in 46ffc92

[kaalibro]

But the extension already supports external directories. I added the ipadapter in extra_model_paths.yaml and it works. Unless I've misunderstood the intent of this PR.

[fofr]

The intent of this PR is just to bring IPAdapters inline with other model types.

[mcmonkey4eva]

This should probably be handled in the IP-Adapter extension?
Or if for whatever reason it's unable to be handled from there, a generic API for it on the comfy side so extension can handle it as-needed.