fix s3 flex checksums for bad memory access

[sbiscigl]

Description of changes:

Currently our S3 integration tests will emit a valgrind warning for using unitialized memory when using a "bad" pre-calculated checksum that is not of Base64 format. When you execute the code

const auto badDecoded = HashingUtils::Base64Decode("If he came back and wanted you?");
const auto encodedAgain = HashingUtils::Base64Encode(badDecoded);

you will get a valgrind warning. This is due to a bug in our base64 implementation. I fix this in this case by using the CRT implementation of Base64 that fixes this issue. Going forward it would be recommended to use the CRT implentation and not the SDK implementation.

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• Did a review by yourself.
• Added proper tests to cover this PR. (If tests are not applicable, explain.)
• Checked if this PR is a breaking (APIs have been changed) change.
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