- Implement OffsetCommit API (key 8) for consumer offset persistence
- Implement OffsetFetch API (key 9) for consumer offset retrieval
- Add comprehensive offset management with group-level validation
- Integrate offset storage with existing consumer group coordinator
- Support offset retention, metadata, and leader epoch handling
- Add partition assignment validation for offset commits
- Update ApiVersions to advertise 11 APIs total (was 9)
- Complete test suite with 14 new test cases covering:
* Basic offset commit/fetch operations
* Error conditions (invalid group, wrong generation, unknown member)
* End-to-end offset persistence workflows
* Request parsing and response building
- All integration tests pass with updated API count (11 APIs)
- E2E tests show '84 bytes' response (increased from 72 bytes)
This completes consumer offset management, enabling Kafka clients to
reliably track and persist their consumption progress across sessions.