Allow to export work items description widget as CSV

What does this MR do and why?

Includes description widget data when exporting work items as CSV.

related to #379155 (closed)

How to set up and validate locally

1. Use rails console to enable import_export_work_items_csv feature flag.
2. Create a work item with a type that has description widget, for example an issue.
3. Execute the following GraphQL mutation on GraphiQl explorer:

mutation exportWorKitems {
  workItemExport(input: {
    projectPath: "PROJECT_PATH",
  }) {
    errors
  }
}

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.