Add Internal Articles Folders API to Unstable version

- Add folder_id field to internal_article schema (nullable)
- Add folder_id parameter to create internal article request
- Add folder_id query parameter to search endpoint (already present)
- Add new endpoints:
  * GET /internal_articles/folders - List all folders
  * POST /internal_articles/folders - Create a folder
  * GET /internal_articles/folders/{id} - Get folder details
  * PUT /internal_articles/folders/{id} - Update a folder
  * DELETE /internal_articles/folders/{id} - Delete empty folder
  * POST /internal_articles/move_to_folder - Bulk move articles

- Add schemas:
  * internal_article_folder
  * internal_article_folder_list
  * create_internal_article_folder_request
  * update_internal_article_folder_request

This enables folder management for internal articles through the API,
allowing customers to organize their internal documentation.

Relates to: intercom/intercom#472943

Author: sametozcan