LBI: Support an artifact pattern for build/input

When one starts going deep into [logically bound images](https://github.com/containers/bootc/issues/128) I think it starts to fundamentally change the feel/character of the system to be "swarm of images". The "base bootc image" isn't necessarily the most important image at all.

One issue we have today is that we can't do a "dry run" upgrade that would report the total amount of data to be fetched because we don't know until we've fetched the base image what LBIs it references.

In this issue we'd support switching to an OCI artifact instead. This would make the base image far less special - it'd just be another entry in the list of images. A downside is it'd need a custom build process.

It's a lot like how in OCP today the [CVO](https://github.com/openshift/cluster-version-operator) is just a pointer to a lot of other images.