Add historical journald and log export flags to operator debug command

[tehut]

Description

Support requested the ability to retrieve historical journald logs as part of the debug capture returned by nomad operator debug. PR #26178 introduced a MonitorExport() RPC capable of streaming the contents of an agent's log file (if file logging is enabled) or its journald logs.

This PR adds that functionality to operator debug with the inclusion of either a log-file-export=true flag or a -log-lookback=<duration> flag. An agent's historical log capture runs alongside the capture of running logs and results in both monitor.log and monitor_export.log files appearing in each agent's debug capture.

Note to reviewers: I exported a test helper from PR #26178 for this new test case. The helper generates a golden log file for testing against stream results. And since the helper is now exported, I also refactored a few tests from that PR to use it as well since they were using the same mechanism.

Testing & Reproduction steps

To run locally:

1. Build branch
2. Run a debug command with one of the historical log operators e.g nomad operator debug -duration=10s -interval=5s -log-file-export=true
3. Unzip the debug archive to display a monitor.log and monitor_export.log file for each agent in the cluster.
   

Example below

Links

Requested in: NMD-141

Contributor Checklist

• Changelog Entry If this PR changes user-facing behavior, please generate and add a
  changelog entry using the make cl command.
• Testing Please add tests to cover any new functionality or to demonstrate bug fixes and
  ensure regressions will be caught.
• Documentation If the change impacts user-facing functionality such as the CLI, API, UI,
  and job configuration, please update the Nomad website documentation to reflect this. Refer to
  the website README for docs guidelines. Please also consider whether the
  change requires notes within the upgrade guide.

Reviewer Checklist

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  backporting document.
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  in the majority of situations. The main exceptions are long-lived feature branches or merges where
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  within the public repository.

[aimeeu]

Thanks for updating the docs! I left a few style guide suggestions.

[aimeeu]

Suggested change

	configure that monitor. Nomad will return an error if the agent does not
	configure that monitor. Nomad returns an error if the agent does not

[aimeeu]

Suggested change

	This flag is only available on Linux systems, see the `-log-file-export`
	This flag is only available on Linux systems. Refer to the `-log-file-export`

[tgross]

LGTM!

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