Kafka gateway: Kafka DescribeGroups and ListGroups admin APIs

[ryerraguntla]

Part of #3560.

Summary

Implement DescribeGroups (15) and ListGroups (16) for observability and kafka-consumer-groups.sh compatibility.

Depends on

• Kafka gateway: Kafka FindCoordinator, JoinGroup, SyncGroup, and Heartbeat APIs #3541 — consumer group state exists
• Kafka gateway: Kafka gateway Phase 1 — docker-compose, E2E tests, and quick-start docs #3539 — Phase 1

Scope

• ListGroups: active groups on coordinator
• DescribeGroups: members, assignment, state (Stable, CompletingRebalance, etc.)
• Integration test with running consumer group

Acceptance criteria

• Admin can inspect group membership via Kafka CLI tools

References

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