* s3api: fix AccessDenied by correctly propagating principal ARN in vended tokens
* s3api: update TestLoadS3ApiConfiguration to match standardized ARN format
* s3api: address PR review comments (nil-safety and cleanup)
* s3api: address second round of PR review comments (cleanups and naming conventions)
* s3api: address third round of PR review comments (unify default account ID and duplicate log)
* s3api: address fourth round of PR review comments (define defaultAccountID as constant)