Support per-task RestartPolicy

[notnoop]

Here, we add support for having a per-task RestartPolicy configured by user. With Task Lifecycle coming in the horizon, tasks may need to have different RestartPolicies, e.g. Sidecar or one-off tasks should have different restart policies from main tasks.

In this iteration, the task restart policy inherits and is merged with the TaskGroup it is in, a common Nomad pattern for other stanzas that apply in many levels:

# assuming Task Group restart policy is
restart {
  interval = "30m"
  attempts = 2
  delay    = "15s"
  mode     = "fail"
}

# and individual task group is
restart {
      attempts = 5
}

# then the effective individual task group policy is
restart {
  interval = "30m"
  attempts = 5
  delay    = "15s"
  mode     = "fail"
}

One outstanding question for the Task Lifecycle PR is to whether to have different default policies for one-off or sidecar tasks. It's outside of scope for this PR.

[github-actions]

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