fix(s3): wrap credentials with NewCredentialsCache

[kakao-tars-case]

Summary

• Wrap AWS credentials with aws.NewCredentialsCache() to cache resolved credentials and avoid redundant credential resolution on every API call
• Also cache credentials before passing to STS AssumeRoleProvider in IRSA + AssumeRole flow

Details

Without NewCredentialsCache, each S3 API call may trigger a fresh credential resolution (e.g., hitting IMDS or STS endpoint). This adds latency and can cause throttling under high request volume.

Validation

• go test ./pkg/storage/ -v — all unit tests pass
• go build ./... — build succeeds

[Slach]

thanks for contibution