Came up in a discussion in the block-themes meeting
Related to #24634 and #22081

If we want FSE themes to be able to build nice headers using the post's featured image, we need to allow the use of dynamic data as properties for blocks.
For example if we want to do something like this:

we'll need to allow the group block to have a background set to the featured image.

Ideally we'd have a method abstract enough to allow using any kind of dynamic data on many blocks, but for the purposes of FSE, background-images in group blocks are a good place to start.