Add WorkItem description widget to GraphQL API

What does this MR do and why?

Implements the first widget for work item. Adds widget to the work item type (read only)

Related to #334809 (closed)

How to set up and validate locally

1. Run bundle exec rails console
2. Run Feature.enable(:work_items)
3. Execute a GraphQL query like

{
  workItem(id: "gid://gitlab/WorkItem/549") {
    id
    widgets {
      type
      ... on WorkItemWidgetDescription {
        markdown
        html
      }
    }
  }
}

MR acceptance checklist

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