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>

.github/workflows/gotgt.yml

@@ -39,6 +39,9 @@ jobs:
     - name: Unit Test
       run: go test -v ./pkg/... ./mock/...
 
+    - name: S3 Backend Store Test
+      run: go test -tags s3integration -v ./pkg/scsi/backingstore/s3store/
+
     - name: Function test
       run: |
         dd if=/dev/zero of=/var/tmp/disk.img bs=1024 count=102400