Quadlet .artifact: Add AutoUpdate key + container/pod restart mechanism

[M-PERSIC]

Hi!

My ideal setup for a Quadlet-oriented sever would include config files stored as OCI artifacts (separation of concerns + allow for quick config file tests). Thanks to mount support (#25397), I understand it is possible to mount artifacts to my Quadlet containers, however there is currently no method to auto-update the artifact as a .artifact Quadlet. Essentially, if we could support:

[Artifact]
# ...
AutoUpdate=registry

as well as provide a mechanism to restart any containers (or even the whole pod) when the mounted artifact auto-updates. This would help support immutable machines which we do not touch following provisioning. For now, I can envision including the config files in a bootc image, but this would lead to temporary downtime for every update.