Add assignees to work item widgets in GraphQL

What does this MR do and why?

Allows querying for a work item's assignees

Sample query:

query workItem($id: WorkItemID!) {
  workItem(id: $id) {
    ...WorkItem
    __typename
  }
}

fragment WorkItem on WorkItem {
  id
  title
  state
  description
  workItemType {
    id
    name
    __typename
  }
  userPermissions {
    deleteWorkItem
    updateWorkItem
    __typename
  }
  widgets {
    ... on WorkItemWidgetDescription {
      type
      description
      descriptionHtml
      __typename
    }
    ... on WorkItemWidgetAssignees {
      type
      allowsMultipleAssignees
      assignees {
        nodes {
          id
          avatarUrl
          name
          username
          webUrl
        }
      }
      __typename
    }
    __typename
  }
  __typename
}

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

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

Related to #334810 (closed)