Add support for rejecting invalid inputs

[KanishAnand]

Adds support to reject storing invalid inputs in corpus. There is an ongoing discussion about how to support this while preserving backward compatibility.

This PR adds two ways to reject invalid inputs:

1. Defining LIBPROTOBUFMUTATOR_ENABLE_RETURN_CODE via the build system or #defineing it before including libfuzzer_macro.h.
2. Using the DEFINE_PROTO_FUZZER_RET macro instead of DEFINE_PROTO_FUZZER.

[hartwork]

Hi @KanishAnand this seems like an important fix, thanks for your work on the topic! 👍 Personally I believe one of the two approaches being implemented simultaneously here — the build system version — makes things overly complex and "magic" without a strong enough return in value: One way, the new macro way, seems enough. So my vote would be to drop the everything LIBPROTOBUFMUTATOR_ENABLE_RETURN_CODE from the pull request but ideally after @vitalybuka has confirmed buy-in in the pull request in general and this very direction too, so that you don't waste your time. PS: I'm not a maintainer here.

[hartwork]

@vitalybuka do you have any thoughts on ⬆️ ?