Backport of logs: allow disabling log collection in jobspec into release/1.5.x

[hc-github-team-nomad-core]

Backport

This PR is auto-generated from #16962 to be assessed for backporting due to the inclusion of the label backport/1.5.x.

The below text is copied from the body of the original PR.

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Some Nomad users ship application logs out-of-band via syslog or the like. For these users having logmon (and docker_logger) running is unnecessary overhead. Allow disabling the logmon and pointing the task's stdout/stderr to /dev/null.

This changeset is the first of several incremental improvements to log collection short of full-on logging plugins. The next step will likely be to extend the internal-only task driver configuration so that cluster administrators can turn off log collection for the entire driver (as described in #15686 and #14636)

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Fixes: #11175

Notes to reviewers:

• In addition to the unit tests here, I've verified this works as expected with docker and raw_exec tasks, with and without the logs.enabled set, including requiring destructive updates to the alloc if we change the value.
• Although we have an existing disable_logging_collection field for the driver, I went with enabled here to be more consistent with other jobspec fields and our inclusive language guidelines. I suspect most folks won't touch it, in any case.
• Once this is merged I'll open a follow-up issue for disabling the logs UI if LogConfig.Enabled = false. But I've verified it doesn't cause any weird crashing behavior, the panel is just empty.