feat(client): Abstract client-side data transmission for async and batch optimization

[xiaguan]

Overview

This PR introduces a comprehensive abstraction layer for client-side data transmission, laying the foundation for future asynchronous interfaces and batch optimizations.

Technical Details

Architecture Changes

// Before: Direct transfer engine calls
engine_.submitTransfer(batch_id, requests);

// After: Abstracted through TransferSubmitter
auto future = transfer_submitter_->submit(handles, slices, op_code);
ErrorCode result = future->get();

Future Roadmap

This abstraction enables several planned optimizations:

1. Thread Pool Integration: Ready for introducing thread pools for parallel processing
2. Async Memcpy: Foundation for making memory copy operations asynchronous
3. Advanced Batching: Enhanced batch processing
4. Performance Monitoring: Built-in hooks for transfer performance metrics