Add `@optionalAuth` to all S3 operations (#2955)

In #2907, I created an allow list of S3 operations to add
`@optionalAuth` to, but this turns out to be too restrictive, as seen in
https://github.com/awslabs/aws-sdk-rust/issues/878. This PR restores the
original middleware behavior of allowing optional auth for all S3
operations.

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