[yaml] allow logging bytes in LogForTesting

[Polber]

LogForTesting will fail with a JSON serialization error for fields that are of type bytes (since JSON has no such concept), so transforms like ReadFromPubSub (using RAW format) will fail. This refactor will print the bytes as-is to help with debuggability without the use of an intermediate MapToFields (which may not be obvious if the user does not know the upstream transform insert a bytes field into the output Row).

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[Polber]

R: @robertwb

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