Kafka gateway: Kafka FindCoordinator, JoinGroup, SyncGroup, and Heartbeat APIs #3541

Part of #3560.

Summary

Implement Kafka consumer group membership protocol (API keys 10, 11, 12, 14) in the gateway coordinator.

Motivation

Phase 2 goal: full consumer group semantics — join, assignment, heartbeat, rebalance. Required for kafka-console-consumer.sh --group.

Depends on

• Kafka gateway: Kafka gateway Phase 1 — docker-compose, E2E tests, and quick-start docs #3539 — Phase 1 complete
• Kafka gateway: Design Consumer offset persistence strategy for Kafka gateway Phase 2 #3540 — offset persistence design approved

Scope

• FindCoordinator (10) — return gateway as coordinator
• JoinGroup (11) — member registration, leader election (single-bridge coordinator)
• SyncGroup (14) — partition assignment to members
• Heartbeat (12) — session timeout, trigger rebalance on missed heartbeats
• Assignment strategies: document supported (range, round-robin, sticky) or subset
• In-memory or persisted group state per Kafka gateway: Design Consumer offset persistence strategy for Kafka gateway Phase 2 #3540

Acceptance criteria

• Two consumers same group get disjoint partition assignment
• Heartbeat timeout evicts member and triggers rebalance

References

• Rollout plan: feature - Order of Implementation discussion of kafka to iggy bridge #3253 (comment)