Support custom shutdown script

[everpeace]

When using preemptible instance and the instance was preempted, we would like to send some notification or add a comment to the PR which informs the runner was preempted and it might require to re-run the jobs.

ref: https://cloud.google.com/compute/docs/shutdownscript

[ravwojdyla]

@everpeace oh - that would be great to have! Would love a PR for this.