Allow unaltered payload response from codec

[travismcchesney]

What was changed

This change allows for codecs to return unaltered payloads, without the need to explicitly
clone them first.

Why?

It's a bit unintuitive that one can't simply return the unaltered payload, and there are a
handful of sample code snippets that do exactly this.

Checklist

1. Closes [Bug] Returning original unaltered payload from codec causes error #417

2. How was this tested:
   A new codec was added to the unit tests to exercise the code with a codec that simply
   returns the unaltered payload. The new unit testing will fail with existing code, but pass
   with the new changes.

3. Any docs updates needed?
   Probably not required, but if desired, the codec section could be updated to explicitly
   state that it's possible to return an unaltered payload: https://docs.temporal.io/develop/dotnet/converters-and-encryption#custom-payload-codec

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[cretz]

Looks great, thanks! May need to click that button to sign CLA