feat: add S3-compatible object storage backend

Add a new backend store that enables iSCSI targets backed by
S3-compatible object storage (AWS S3, MinIO, Ceph RGW, etc.).

The implementation uses a chunked storage strategy where the virtual
block device is divided into fixed-size chunks (default 4 MiB), each
stored as an independent S3 object. This enables efficient random
read/write access on top of object storage.

Key features:
- Chunked storage with configurable chunk size
- Sparse device support (unwritten chunks treated as zeros)
- Concurrent multi-chunk I/O via errgroup
- Per-chunk locking for safe read-modify-write
- AWS SDK v2 with default credential chain
- In-process gofakes3 test server (no Docker needed)
- 12 unit tests + 2 integration tests

Also updates CI workflow to run S3 backend tests and updates
README with S3 backend documentation.

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

pkg/api/types.go

@@ -407,6 +407,9 @@ type SCSILu struct {
 
 	PerformCommand CommandFunc
 	FinishCommand  func(*SCSITarget, *SCSICommand)
+
+	// BackendConfig holds backend-specific configuration (e.g., *config.BackendStorage)
+	BackendConfig interface{} `json:"-"`
 }
 
 type LUNMap map[uint64]*SCSILu