Create verification status widget for requirements

Allow to read a requirement verification status through a widget on GraphQL. The functionality is similar to the description widget that is already implemented. We should be able to query a requirement verification status with a query like:

query workItem {
  workItem(id: "gid://gitlab/WorkItem/469") {
    title
    widgets {
      type
      ... on WorkItemWidgetVerificationStatus {
        verificationStatus
      }
    }
  }
}

There is a draft MR that implements this feature and can be used as a starting point. It does not take care of potential N+1 queries, this is something that will probably require some attention.