Add a new backend for Darkibox (https://darkibox.com/), an
XFileSharing-based video hosting platform.

This implements the core rclone operations using the Darkibox REST API
(https://darkibox.com/api.html):

- List files and directories (with pagination)
- Upload files (via upload server)
- Download files (via direct link API with quality selection)
- Create and delete directories
- Delete files
- Move and rename files
- Account info (About)

Authentication is via API key, available from the darkibox API page.

Resolves rclone#7360

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>