Update `add_child_work_item` so that we can create child items in batches/bulk

[danhellem]

Summary

Update add_child_work_item tool so that we can add several child work items to a parent without having to do one at a time.

Accept an array of work items with those same params

Rules

1. Adhere strictly to existing project standards and coding conventions.
2. Ensure each tool exposes all API parameters (required and optional).
3. Use the official Azure DevOps Node API to interact with the APIs.

Special treat

If you follow the rules, you'll get candy!