install: Add `--pull` for LBIs

Today for install we require the base image *and* LBIs to be in the source root container storage.

Having them required to be in the same container storage I think is probably the correct default as it means `bootc install` is not itself ever in the business of pulling anything.

But this hurts ergonomics for the `to-existing-root` type cases because almost always LBIs won't be present and it's logistically annoying. I think we should add something like `bootc install --pull` which would default to automatically fetching LBIs to the *target* root.

Also as I mentioned elsewhere I think it'd also be useful to have something like `bootc image list --type=logical` to start which would print the images so one could do this:

```
podman pull quay.io/examplecorp/bootc:latest
podman run --rm quay.io/examplecorp/bootc:latest bootc image list --type=logical | xargs podman pull
podman run --privileged --rm quay.io/examplecorp/bootc:latest bootc install to-existing-root
```

Note that in this flow it's the host `podman` that's responsible for fetching images always consistently so we pick up whatever pull secrets (e.g.) are needed from there.

If we have the `bootc install` in the container image need to pull, then suddenly we're in the problem domain of needing to proxy all the pull secret configuration, which is something I'd hoped to avoid. OTOH for cases that embed the pull secret in the image sufficient for the target registry, that's fine.