PARQUET-1903: Improve Parquet Protobuf Usability

[belugabehr]

Make sure you have checked all steps below.

Jira

• [X] My PR addresses the following PARQUET-1903 issues

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• [X] My PR adds the following unit tests OR does not need testing for this extremely good reason:

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[dossett]

@belugabehr This looks really good! I'm probably going to be working a lot with protobuf + parquet in the future, so I'm happy to see changes like this. I'm not a parquet committer, but I definitely give this a non-binding +1.

[belugabehr]

@dossett Thanks. There's a bunch more I'd like to add, but this is already a lot.

Also check out:

https://issues.apache.org/jira/browse/PARQUET-1914

[dossett]

That's a good one too @belugabehr ! I posed this question on the parquet dev list about proto3 if you are interested:

http://mail-archives.apache.org/mod_mbox/parquet-dev/202009.mbox/%3CCAMgkoMJmLX%2BNc0-qnMLqkx6aGL1wW%3DMeJkGGTONG3ypmj9LUyw%40mail.gmail.com%3E

[dossett]

I've been in parquet-protobuf code lately and these seem like good changes. @belugabehr I'd be happy to pick this PR up if you like and address some of the minor comments. The biggest concern was a possible breaking change, but I don't think that's the case and tried to explain in the PR comments.