Kafka gateway: Kafka DeleteTopics, DescribeConfigs, and AlterConfigs admin APIs

[ryerraguntla]

Part of #3560.

Summary

Phase 4 admin: topic deletion and config describe/alter (limited support).

API keys

• DeleteTopics (20) → delete Iggy stream + topic
• DescribeConfigs (32) → return topic/broker configs
• AlterConfigs (33) → modify configs where mappable to Iggy

Depends on

• Kafka gateway: Kafka gateway Phase 1 — docker-compose, E2E tests, and quick-start docs #3539 — Phase 1

Scope

• DeleteTopics with per-topic errors
• DescribeConfigs: return sensible defaults + Iggy-mapped settings
• AlterConfigs: support subset; reject unknown with NOT_CONFIGURABLE
• kafka-topics.sh --delete integration test

Acceptance criteria

• kafka-topics.sh works against gateway for create/delete/describe

References

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