Destination S3: check to verify prefix access instead of full-bucket access

[itaiad200]

closes #3989

What

The S3Destination connector tries to write to the bucket root as part of the connector checks.
Users shouldn't be forced to give the connector full access to the bucket, if they configured a prefix within the bucket.

How

Perform the check on an object under the prefix in the bucket.

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Pre-merge Checklist

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