Enable IMDSv1 fallback for retrieving credentials from instance metadata.

Useful for feature parity with Arrow object_store: https://github.com/apache/arrow-rs-object-store/blob/main/src/aws/builder.rs#L141-L142

Read the referenced Arrow object_store implementation at src/aws/builder.rs around lines 141-142, then locate the repository's current instance-metadata credential retrieval path. Implement the IMDSv1 fallback alongside the existing behavior and add or update coverage showing credentials can be retrieved when the newer metadata flow is unavailable.

Written by the indexing model from the issue text.