Add support for post cleanup script in runner configuration

What does this MR do?

This MR adds support for a post_cleanup_script, analog to the post_build_script

Why was this MR needed?

Currently there is no way to customize the cleanup steps needed after a jobs has run. For us specifically this includes performing sanity checks on the VM like disk usage, reachability of the docker socket and so on.

What's the best way to test this MR?

Create a script and add it to the config.toml under post_cleanup_script. Register the runner and have it run a job. Verify that the script has executed as expected.

What are the relevant issue numbers?

!28144