Kafka gateway: Kafka ListOffsets API (key 2) — earliest/latest via Iggy high watermark #3537

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

• Kafka gateway: Build Gateway to Iggy for existing Kafka users - Create a TCP Listener for Kafka users #3421
• Kafka gateway: Kafka gateway IggyBridge core — SDK client and stream/topic mapping #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: feature - Order of Implementation discussion of kafka to iggy bridge #3253 (comment)