Local package layering story with bootc & dnf5

For Fedora CoreOS and Fedora Atomic Desktops, we'll need support for local package layering. Right now, this is achieved using rpm-ostree client side package layering.

We should discuss the user experience and the design for package layering in a bootc only world (with no rpm-ostree).

Related links:

• https://dnf.readthedocs.io/en/latest/command_ref.html#shell-command-label
• https://github.com/ublue-os/main/blob/main/packages.json