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

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

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

Start by tracing the Kafka gateway's admin API handling and reviewing the Phase 1 work in #3539. Exercise DeleteTopics, DescribeConfigs, and AlterConfigs through the gateway, then run the `kafka-topics.sh` integration flow. Done means create, delete, and describe work, mapped configs are returned or changed, and unsupported settings receive NOT_CONFIGURABLE.

Written by the indexing model from the issue text.