List view: Add image thumbnails to additional image attribute blocks

[richtabor]

While #53381 adds support for including image thumbnails in List view for core/image and core/gallery blocks, let's extend that functionality to other blocks with image attributes:

• Cover
• Media & Text
• Featured Image (maybe)

It may be good to think through an API that goes beyond hard-coding core blocks, as @mtias mentions in #53381 (comment).

Visual

cc @andrewserong

[andrewserong]

Thanks for writing this up, Rich! I've added it to the tracking issue I'm working from (#49563) 👍

[mtias]

Another angle to look at scaling this to other blocks is connecting with the work on image background support. Ideally when you add support for background images on a group you also get this working without having to opt-in, etc.

[andrewserong]

Just moving this one to the 6.5 column as I don't believe it's urgent for 6.4, since we're already handling thumbnails for images and gallery blocks. Let me know if anyone thinks it's more urgent than that.