ore/netio: support splitting streams into tx/rx halves

[teskje]

This PR adds support for splitting mz_ore::netio::Stream into a read and a write half, so reading from and writing to a connection can be done independently.

Split out of the work that adds the new cluster transport protocol (#32330).

Motivation

• This PR adds a known-desirable feature.

Part of https://github.com/MaterializeInc/database-issues/issues/5038

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.

[teskje]

TFTR!