Storefront: Display variant templates as image thumbnails

Differs from https://roadmap.meshnordic.ai/p/storefront-display-variant-images-as-thumbnails since an image is set ONCE per variant template value, instead of setting an image for each product's variant.

• I.e if variant template "Texture" has 10 options, you set 10 images.

• Instead of, if you have 500 products with 10 textures each = 500x10 = 5000 images.