scalability framework: Introduce password + sasl connection tests

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With password and 1500 connections we ran into connection timeouts, ouch! https://buildkite.com/materialize/nightly/builds/14376 Same with 1024: https://buildkite.com/materialize/nightly/builds/14377 Successful run with 512: https://buildkite.com/materialize/nightly/builds/14381#019af825-fdcf-4b93-bf2d-76517a53d7b1 Rebased to include https://github.com/MaterializeInc/materialize/pull/34349: https://buildkite.com/materialize/nightly/builds/14384

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.

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Ready for review!