Include plugin own settings into the downloaded yaml

[proski]

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• [x] Link to any upstream changes that might be required (for example Jenkins Core pull request)

Feature Request

I'm using following section in the beginning of jenkins.yaml:

configuration-as-code:
  version: "1"
  deprecated: "warn"
  restricted: "warn"
  unknown: "warn"

However that section is not present in the downloaded jenkins.yaml. Please include it. Rationale:

• The settings become discoverable.
• Issues like #1581 won't be filed again.
• Users won't need to restore the missing section if they keep jenkins.yaml under version control.
• This plugin would do what it wants other plugins to do, i.e. save their settings.