When access-key / secret-key are left blank, fall back to the AWS
DefaultCredentialsProvider chain so that deployments on EC2, ECS,
and EKS can authenticate via instance profile, task role, or IRSA
without static credentials.
- Extract buildCredentialsProvider() in S3StorageService
- Add sts dependency for Web Identity Token (EKS) support
- Add unit tests for credential provider selection
- Update storage-spi docs (zh + en) and env example