Implement Smart Object Storage API (SOSAPI) support for Veeam integration.
- Add s3api_sosapi.go with XML structures and handlers for system.xml and capacity.xml
- Implement virtual object detection and dynamic XML generation
- Add capacity retrieval via gRPC (to be optimized in follow-up)
- Include comprehensive unit tests covering detection, XML generation, and edge cases
This enables Veeam Backup & Replication to discover SeaweedFS capabilities and capacity.
