What is the purpose of this change?

This PR adds iCloud Photos support to the existing iclouddrive backend, creating a unified iCloud backend that allows users to access both iCloud Drive and iCloud Photos through a single backend with service selection.

Key changes:

• Unified Backend: Users can now select "Drive" or "Photos" when configuring the iclouddrive backend, eliminating the need for a separate icloudphotos backend
• Shared Authentication: Both services use the same authentication flow and session management, reducing code duplication
• Photos Functionality: Full read-only access to iCloud Photos including:
  • Library and album listing with accurate file counts
  • Smart albums support (All Photos, Videos, Favorites, etc.)
  • Photo and video downloading with range request support
  • Proper metadata handling (file sizes, modification times)
• Clean Architecture: Router-based design that delegates to service-specific implementations while maintaining separation of concerns
• Backward Compatibility: Existing iclouddrive configurations continue to work unchanged (defaults to Drive service)

Technical Implementation:

• Added service configuration option to select between "drive" and "photos"
• Created icloud.go as the main router handling authentication and service routing
• Added Photos implementation to icloudphotos.go within the iclouddrive package
• Shared API client code between Drive and Photos (api/client.go, api/session.go)

Was the change discussed in an issue or in the forum before?

#7982

Checklist

• I have read the contribution guidelines.
• I have added tests for all changes in this PR if appropriate.
• I have added documentation for the changes if appropriate.
• All commit messages are in house style.
• I'm done, this Pull Request is ready for review :-)