Kafka gateway: Kafka ListOffsets API (key 2) — earliest/latest via Iggy high watermark

Part of #3560.

## Summary

Implement Kafka **ListOffsets (API key 2)** using `IggyBridge::high_watermark()` and earliest offset resolution.

## Motivation

Clients call ListOffsets before Fetch to resolve `-1` (latest) and `-2` (earliest) timestamps/offsets.

## Depends on

- #3421
- #3533 — IggyBridge core

## Scope

- [ ] Support timestamp types: earliest, latest (scoped versions)
- [ ] Per-partition offset in response; error code per partition on failure
- [ ] Tests: empty topic, topic with messages, invalid partition

## Acceptance criteria

- [ ] Java/librdkafka client can resolve offsets before fetch
- [ ] ListOffsets consistent with Fetch high-watermark

## References

- Rollout plan: https://github.com/apache/iggy/discussions/3253#discussioncomment-17181030