Add simple example of before/after reboot annotations

[invidian]

We have e.g. https://github.com/flatcar-linux/flatcar-linux-update-operator/blob/53f08043e320c853940ed7b4c126c7b72af1af00/examples/reboot-annotations/after-reboot-daemonset.yaml right now, but it only annotates the node right away, which is not very useful, as it does not allow implementing any logic.

We should change those to:

• Use an agent Service Account so hook has appropriate permissions to update the Node object.
• Use shell + kubectl to be able to write some simple logic for annotating nodes. Something like this should be possible:
  while true; do kubectl annotate node $NODENAME --overwrite=true has-attached-volumes=$(if [ $(kubectl get volumeattachment | grep $NODENAME | wc -l) -gt 0 ]; then echo true; else echo false; fi); sleep 30; done (to workaround Ensure update-agent waits for all volumes to be detached before rebooting #30).