Add `bootc uninstall`

One thing I'd like to support is having OS vendors have e.g. cloud images that start out in bootc mode, but if e.g. there's some app/workload that isn't compatible, they can run say: `bootc uninstall` and revert back to a classic mutable package-based workflow.

One implementation detail I think we can make happen here is that this could happen "live":

- reflink copy all the objects back to `/`
- Clone `/etc` and `/var` back to `/`
- remount the rootfs back to that
- On reboot `bootc-destructive-cleanup.service` would do the *opposite* of what it does today - remove the old bootc state