Add hasChildren field to work items hierarchy widget

Related to #383538 (closed)

What does this MR do and why?

Adds a new field to work items hierarchy widgets that indicates if the work item has children.

Example query:

query {
  workItem(id: "gid://gitlab/Issue/113355760") {
    widgets {
      ... on WorkItemWidgetHierarchy {
        hasChildren
      }
    }
  }
}

example response

{
  "data": {
    "workItem": {
      "widgets": [
        {},
        {},
        {},
        {},
        {},
        {
          "hasChildren": true
        },
        {},
        {},
        {}
      ]
    }
  }
}

This new field uses BatchLoader::GraphQL to batch results for multiple work items with the following query:

SELECT "work_item_parent_links"."work_item_parent_id"
FROM   "work_item_parent_links" 
WHERE  "work_item_parent_links"."work_item_parent_id" IN (115219866, 113355760) 
GROUP  BY "work_item_parent_links"."work_item_parent_id" 

Query plan (internal): https://console.postgres.ai/gitlab/gitlab-production-tunnel-pg12/sessions/13644/commands/47894

MR acceptance checklist

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