add configurable number of shards to compare_and_set bench

[ptravers]

Motivation

we want to test how persist behaves with 2000 shards doing compare_and_set.

request from reviewers of: https://www.notion.so/materialize/High-CPU-Usage-Emulator-1f113f48d37b80c395def016bea16b0d

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.