uid/gid tracker (sysusers, systemd json, etc.)

Let's move https://github.com/coreos/fedora-coreos-tracker/issues/155 here, as it's really a generic issue that applies to all Fedora image based systems, and I think all of those should derive from bootc core.

(Although looking at this from a generic lens, while it's mostly only Ignition that is doing system provisioning from the initramfs, there are related issues around Anaconda %post that may touch non-root owned content, etc.)