Tooling to import images from the TUF repo

[yeazelm]

What I'd like:
bottlerocket-os/bottlerocket#2486 covers the modify-image flow where modifying a pre-built image is faster or more advantageous than building from source. There are really two distinct pieces here, the modification of images vs downloading built images from the TUF repo and placing them in the working directory for modification. This issue covers importing those images.

Ideally this would be a make target such as cargo make import-images that takes a variant and version and downloads from a configured TUF repo. There is likely work to also have the kmod-kit and migrations also downloaded but I think we could scope that to yet a different issue if needed since modifying the images is really the only thing strictly needed.

I wrote a bash version of this target but ideally, this should go in pubsys to keep all the logic around where these images should be in one place.

Any alternatives you've considered:
Building from source, which sort of defeats the point of bottlerocket-os/bottlerocket#2486