Support rebase in the rpm-ostree plugin
There's a bit of a terminology clash here. "rebase" is what rpm-ostree calls switching to a new OS release - I guess thats "upgrade" in gnome-software, but "upgrade" means to just update to the latest update within an OS release stream in rpm-ostree terminology.
In any case, we should support "rebase", and reuse our existing UX for switching to a newer OS release.