[Feature Request]: Allow usage of thumbnailVariant in custom_nodes

[lerignoux]

Is there an existing issue for this?

• 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 #3682

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

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[lerignoux]

Since there was little follow up I guess a different solution was preferred.
I tried to unify the custom nodes management with the core custom nodes index.json
See Merge Request

Also proposing the documentation update with these changes;
see Merge Request