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>

cmd/daemon.go

@@ -32,6 +32,7 @@ import (
 	_ "github.com/gostor/gotgt/pkg/port/iscsit"
 	"github.com/gostor/gotgt/pkg/scsi"
 	_ "github.com/gostor/gotgt/pkg/scsi/backingstore"
+	_ "github.com/gostor/gotgt/pkg/scsi/backingstore/s3store"
 )
 
 func newDaemonCommand() *cobra.Command {