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/scsi/backingstore/common.go

@@ -63,7 +63,7 @@ func parseStoragePath(path string) (backendType, filePath string) {
 		possibleType := path[:idx]
 		// Check if it's a known backend type
 		switch possibleType {
-		case "file", "iouring", "ceph", "null", "RemBs":
+		case "file", "iouring", "ceph", "null", "RemBs", "s3":
 			return possibleType, path[idx+1:]
 		}
 	}