This PR adds support for agent shutdown lifecycle states andshutdown_script.

It (currently) relies on terraform provider abilities to gracefully shut down. For instance with the docker provider it can be controlled via:

resource"docker_container""workspace" {destroy_grace_seconds=30stop_timeout=30stop_signal="SIGINT"}

The agent can handle common signals likeSIGHUP,SIGINT,SIGTERM. If need be, more signals could be handled.

Builds upon#5171
Fixes#4677

Note:

• In hindsight, I should probably have called the lifecycle statesstopping,stop_timeout,stop_error,stopped, yay or nay to make this change?