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/config/config.go

@@ -114,6 +114,16 @@ type BackendStorage struct {
 	NumaNode int `json:"numaNode,omitempty"`
 	// IoUringQueueDepth specifies the io_uring queue depth (0 for default)
 	IoUringQueueDepth uint32 `json:"ioUringQueueDepth,omitempty"`
+	// DeviceSize specifies the virtual device size in bytes (used by S3 backend)
+	DeviceSize uint64 `json:"deviceSize,omitempty"`
+	// S3ChunkSize specifies the chunk size in bytes for S3 backend (default 4MiB)
+	S3ChunkSize int64 `json:"s3ChunkSize,omitempty"`
+	// S3Endpoint specifies a custom S3 endpoint URL (for MinIO, etc.)
+	S3Endpoint string `json:"s3Endpoint,omitempty"`
+	// S3Region specifies the AWS region
+	S3Region string `json:"s3Region,omitempty"`
+	// S3ForcePathStyle uses path-style addressing (required for MinIO)
+	S3ForcePathStyle bool `json:"s3ForcePathStyle,omitempty"`
 }
 
 type ISCSIPortalInfo struct {