[wip] Allow reading back data in structured order

[bkirwi]

Motivation

Part of #24830 - allows reading back data in the structured-data order.

Before this change, we could configure Persist to compact data in the new structured-data order, but when we were consolidating at read time we'd reorder it back to the codec order. This is inefficient, and we'd like to allow callers to rely on the sorted order of data in the future for things like PARTITION BY.

Tips for reviewer

This uses the same ordering flag as the compaction side, which is a little weird: when we first flip the flag, we'll start using the new order for the fast-path peeks before there's any meaningful percentage of batch parts in the new order. It's possible we'll want to add fancier knobs for the true rollout, but this is enough to toggle things in CI / staging for now.

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).
• If this PR includes major user-facing behavior changes, I have pinged the relevant PM to schedule a changelog post.