feat: add S3-compatible object storage backend

[carmark]

Summary

• Add S3-compatible object storage as a new backend store for iSCSI targets
• The virtual block device is divided into fixed-size chunks (default 4 MiB), each stored as an independent S3 object, enabling efficient random read/write on top of object storage
• Compatible with AWS S3, MinIO, Ceph RGW, and other S3-compatible services
• Add CI test step using in-process gofakes3 server (pure Go, no Docker dependency)
• Update README with S3 backend documentation, configuration reference, and performance considerations

Changes

New files:

• pkg/scsi/backingstore/s3store/s3store.go - Core S3 backend implementation
• pkg/scsi/backingstore/s3store/s3store_test.go - 12 unit tests with in-memory fake S3 client
• pkg/scsi/backingstore/s3store/integration_test.go - 2 integration tests using gofakes3
• examples/config-s3.json - Example S3 backend configuration

Modified files:

• pkg/api/types.go - Add BackendConfig field to SCSILu for backend-specific config
• pkg/config/config.go - Add S3 configuration fields (endpoint, region, chunk size, etc.)
• pkg/scsi/lun.go - Thread config to backend, add s3 to known backend types
• pkg/scsi/backingstore/common.go - Add s3 to path parser
• cmd/daemon.go - Register S3 backend store
• .github/workflows/gotgt.yml - Add S3 backend test step
• README.md - Document S3 backend features and configuration
• go.mod / go.sum - Add AWS SDK v2 and gofakes3 dependencies

Test plan

• 12 unit tests pass (fake S3 client: Open, Read, Write, Unmap, cross-chunk, sparse)
• 2 integration tests pass (full lifecycle with gofakes3 in-process server)
• go build ./... succeeds
• gofmt verification passes
• CI workflow passes with new S3 backend test step

🤖 Generated with Claude Code