persist: add support for columnar decode to s3 bench tool

[danhhz]

This needs a schema and relies on a pretty major hack of hardcoding
lineitem, which is what I've been testing with.

I'm also trying to start standardizing on fetch/parse/decode as the
jargon for s3-to-bytes/bytes-to-arrow/arrow-to-KV respectively.

Motivation

• This PR adds a feature that has not yet been specified.

Tips for reviewer

Checklist

• This PR has adequate test coverage / QA involvement has been duly considered. (trigger-ci for additional test/nightly runs)
• This PR has an associated up-to-date design doc, is a design doc (template), or is sufficiently small to not require a design.
• If this PR evolves an existing $T ⇔ Proto$T mapping (possibly in a backwards-incompatible way), then it is tagged with a T-proto label.
• If this PR will require changes to cloud orchestration or tests, there is a companion cloud PR to account for those changes that is tagged with the release-blocker label (example).
• This PR includes the following user-facing behavior changes:

[danhhz]

Cleaning up old PRs