nvidia: support disabling the nvidia plugin

[notnoop]

This provides a workaround for where nomad does not support the nvidia driver library installed on host. The crash in #8303 is a result of an attempt to fingerprint using a C function that's not resolved, so process crashes. Because this happens before agent is up, the logs are completely hidden.

To disable the plugin, one adds the following to the agent config:

plugin "nvidia-gpu" {
  config {
    enabled = false
  }
}

Fixes #8303

[drewbailey]

lgtm

[tgross]

Implementation here LGTM. Just left one comment worrying about confusing/noisy logging, but if I'm up the wrong tree on that this looks good-to-go.

[github-actions]

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