feat: Autorebase to signed image in ignition example

[exponentactivity]

1. I moved the logic from ucore-autorebase.sh into the service itself, to minimize leftover files.
2. I also added another service to rebase to the signed image, after the initial rebase-and-reboot, just like blue-build/template recommends in the README.
3. I ripped out RemainAfterExit=yes since i couldn't figure out why it was needed.
4. I added --bypass-driver to the rebase commands to bypass this error: (building on ucore-minimal)

Apr 25 19:50:33 localhost.localdomain rpm-ostree[1846]: error: Updates and deployments are driven by Zincati (zincati.service)
Apr 25 19:50:33 localhost.localdomain rpm-ostree[1846]: See Zincati's documentation at https://github.com/coreos/zincati
Apr 25 19:50:33 localhost.localdomain rpm-ostree[1846]: Use --bypass-driver to bypass Zincati and perform the operation anyways

I'm not sure if that is the best way to handle it?
I'm guessing Zincati should be completely disabled since it isn't compatible with OCI images, but haven't gotten that far into the weeds yet.

EDIT: I see Semantic PR would have liked me to add Feat: to the start of the commit message (i think. It doesn't show the whole message, and i can't figure out how to get it to do so).
If you wouldn't mind, i would like to be able to fix this my self, in case of future PRs.

[bsherman]

@exponentactivity I think you resynced your fork and it auto-closed this PR...

Please know I was reviewing it last night and welcome you to re-submit it.

I did plan to do a bit of testing on it myself, but I think it looks great.