stabilization tracker: `bootc image copy-to-storage`

Original PR: https://github.com/containers/bootc/pull/620

This issue tracks the stabilization of `bootc image`, which currently just has one subcommand `bootc image copy-to-storage`.

## Rationale

We want to make it ergonomic and convenient to operate on the booted host image via e.g. `podman`. Today usage of podman defaults to operating on `/var/lib/containers`, a distinct mutable store.

Having the ability to conveniently copy the image to the booted storage aids with https://containers.github.io/bootc/booting-local-builds.html

## Concerns

- Longer term, we want to unify bootc and podman storage; but perhaps `copy-to-storage` can become a no-op
- Would it be better to try to instead expose an API to *read* from the bootc storage, and wrap that in a CLI?