Implement moomoo API connector for OHLCV data

- Added DataConnector base class with OHLCV, InstrumentInfo, Interval
- Implemented MoomooClient with rate limiting, circuit breaker, caching
- Mock mode generates realistic data for development
- Real-time WebSocket subscription support (mock)
- Added examples/demo_moomoo.py showcasing functionality
- Updated requirements.txt with requests, websocket-client, redis, python-dotenv
- Updated README.md with Data Layer documentation
- Added .env.example for configuration

data/__init__.py

@@ -0,0 +1,7 @@
+"""
+Data layer for Quantitative Trading Platform.
+"""
+
+from .connectors.moomoo_client import MoomooClient, create_moomoo_client
+
+__all__ = ["MoomooClient", "create_moomoo_client"]