option to restore eligibility after drain_on_shutdown

The new release of nomad v 1.5.4 intruduces the possibility to auto drain nodes via client config

```hcl
drain_on_shutdown {
deadline = "30s"
force = false
ignore_system_jobs = false
}
```

That works so far so good but have some sideeffects.

1. Everytime if nomad worker restarts over systemd then node is drained and ineligible.
2. Evertime on first boot of the machine the node is ineligible

Systemd restarts are required for example when adding or removing a host volume ( when not using the csi driver ) or changing the config config.

### Suggestion

1. Adding a agent setting that makes the node eligable again in automated way
2. Give the agent the possibility to ignore node drain ( for systemd restart for example)

No files, tests, or entry points are named in the issue. Start by tracing the client `drain_on_shutdown` setting and node eligibility across systemd restart and first boot; the issue needs a decided acceptance criterion for either automatic restoration or ignoring the drain before implementation can be scoped.

Written by the indexing model from the issue text.