[Improvement] Use OpenDAL to abstract underlying storage for knowledge base files

[iziang]

Description:
The Aperag knowledge base needs to support document synchronization from multiple sources (file uploads, emails, Feishu/Lark docs, URLs, etc.). These documents should be stored in Aperag for subsequent processing.

We propose using OpenDAL to abstract the underlying storage implementation. This will:
• Support multiple storage backends (object storage, local filesystem, etc.)
• Enable seamless switching between storage solutions based on user environment
• Maintain consistent operations regardless of storage backend

Key requirements:

1. Implement OpenDAL as the storage abstraction layer
2. Maintain existing synchronization channels
3. Ensure backward compatibility

This abstraction will particularly benefit:
• Cloud deployments (compatible with S3/OSS/COS etc.)
• Hybrid environments (mixed cloud/on-premise)
• Local development setups

[github-actions[bot]]

This issue has been marked as stale because it has been open for 30 days with no activity