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

Today we have [offline updates](https://containers.github.io/bootc/registries-and-offline.html), but it doesn't mix in an obviously easy way with [logically bound images](https://containers.github.io/bootc/logically-bound-images.html).

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)