Add preview URL to API endpoint (/info/[id])

[gregzaal]

Some external integrations ask for image previews to be included in the asset info data.

These URLs are consistent, though we prefer certain image sizes for each asset type in order to reused cached data:

1. HDRIs: https://cdn.polyhaven.com/asset_img/thumbs/[ASSET_ID].png?width=371&height=278
2. Textures: https://cdn.polyhaven.com/asset_img/thumbs/[ASSET_ID].png?width=285&height=285
3. Models: https://cdn.polyhaven.com/asset_img/thumbs/[ASSET_ID].png?width=450&height=300