Add support for multiple thumbnails

[agajdosi]

This would be useful for:

• nodegroup asset type: it enables creators to use the 2nd, 3rd and following thumbnails as simple tutorials.
• geometry nodes based assets and processual assets: to show the varieties/how to use these assets
• colour variant assets: multiple coloured variants can be shown
• 3D print assets: print results and details can be shown

Pros:

• easy to understand concept, users are already experienced with uploading thumbnails
• less data than videos
• easier to create, editing images is easier than capturing and editing video

Cons:

• for complex stuff video can show more details

[PetrDlouhy]

@agajdosi You can sychronize with https://github.com/BlenderKit/BlenderKit-server/issues/1038 on which is @milano-slesarik currently working.