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>

examples/config-s3.json

@@ -0,0 +1,31 @@
+{
+    "storages": [
+        {
+            "deviceID": 2000,
+            "path": "s3:gotgt-bucket/iscsi/disk0",
+            "online": true,
+            "backendType": "s3",
+            "blockShift": 9,
+            "deviceSize": 1073741824,
+            "s3ChunkSize": 4194304,
+            "s3Endpoint": "http://localhost:9000",
+            "s3ForcePathStyle": true
+        }
+    ],
+    "iscsiportals": [
+        {
+            "id": 0,
+            "portal": "127.0.0.1:3260"
+        }
+    ],
+    "iscsitargets": {
+        "iqn.2016-09.com.gotgt.gostor:s3-target": {
+            "tpgts": {
+                "1": [0]
+            },
+            "luns": {
+                "1": 2000
+            }
+        }
+    }
+}