(pre-)Stop/Kill action/command

Currently Nomad supports defining a [kill signal](https://www.nomadproject.io/docs/job-specification/task#kill_signal), and it'd be pretty useful to be able to define pre- and stop/kill actions/commands ( we can already do post-stop via tasks with lifecycle > hook > poststop).

The main use case i see for this is shutting down complex software/tasks that needs actions performed on it for a graceful shutdown, e.g. [ScyllaDB](https://docs.scylladb.com/operating-scylla/procedures/cluster-management/safe-shutdown/) recommend running a command (`nodetool drain`) and then shutting down gracefully before killing the Docker container.

It could also be useful in order to do more graceful drains, for example when doing rolling upgrades (e.g. failing the healthcheck to make the instance inaccessible from Consul/LB before actually shutting it down).

In theory it could be achieved with an additional hook (prestop), but that might cause some issues ( e.g. in ScyllaDB's case, the prestop task would need to contain all the tools and configuration to be able to run commands on the ScyllaDB running in the main task; and it won't work for the specific case, since they recommend shutting down gracefully via supervisord after draining, and i don't think one can call supervisorctl remotely).

Adrian

Start by reviewing Nomad's existing kill_signal behavior and the poststop task lifecycle described in the issue. Define the expected semantics for pre-stop and stop/kill actions, including graceful shutdown and rolling-upgrade draining, then verify the behavior against the ScyllaDB use case.

Written by the indexing model from the issue text.