Support fetching LBIs from `oci` when doing e.g. `bootc switch --transport oci /path/to/ocidir`

[cgwalters]

Today we have offline updates, but it doesn't mix in an obviously easy way with logically bound images.

What I'm thinking about here is that if one provides either an oci or containers-storage transport for bootc, we automatically also look for the digested pull specs for LBIs in that storage. (I think in theory this could apply to oci-archive too since I think it can contain multiple images too but, oci-archive is extra inefficient because tar doesn't have seeking)

[cgwalters]

There's also a strongly related thing here osbuild/bootc-image-builder#622 (comment) that's almost the inverse.

[cgwalters]

And yes for bootc-image-builder it does make sense for us to at least support dynamically pulling LBIs. When we do that, we may as well go to a full model where we also support dynamically fetching the bootc image as well. So basically we re-add pull support there.