- Add PartitionOffsetManager for sequential offset assignment per partition
- Implement OffsetStorage interface with in-memory and SQL storage backends
- Add PartitionOffsetRegistry for managing multiple partition offset managers
- Implement offset recovery from checkpoints and storage scanning
- Add OffsetAssigner for high-level offset assignment operations
- Support both single and batch offset assignment with timestamps
- Add comprehensive tests covering:
- Basic and batch offset assignment
- Offset recovery from checkpoints and storage
- Multi-partition offset management
- Concurrent offset assignment safety
- All tests pass, offset assignment is thread-safe and recoverable
