[Feature Request]: Allow usage of thumbnailVariant in custom_nodes

### Is there an existing issue for this?

- [x] I have searched the existing issues and checked the recent builds/commits

### What would your feature do ?

I would like to be able to use the same thumbnailVariant in custom_nodes as the one used in Comfy core templates.

### Proposed workflow

1. If user provide the right data (naming convention and images)
2. The example workflow thumbnail correctly displays the variant in the example workflow list.


### Additional information

I proposed a MR for this in https://github.com/Comfy-Org/ComfyUI_frontend/pull/3682

Will add documentation in the custom nodes doc if it is acceptable.

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