[DNM] - Expose retry and reconnect backoff settings via LD

[martykulma]

TODO: add validation of settings, local repro with SASL failures.

Exposes librdkafka settings as system settings:

• retry.backoff.ms
• retry.backoff.max.ms
• reconnect.backoff.ms
• reconnect.backoff.max.ms

MZ will provide the default values, which are set based on the current defaults described here

Motivation

exposes settings to help alleviate https://github.com/MaterializeInc/database-issues/issues/9801

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.