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/connectors/__init__.py

@@ -0,0 +1,17 @@
+"""
+Data connectors for various APIs.
+"""
+
+from .base import DataConnector, OHLCV, InstrumentInfo, Interval
+from .moomoo_client import MoomooClient, create_moomoo_client, RateLimiter, CircuitBreaker
+
+__all__ = [
+    "DataConnector",
+    "OHLCV",
+    "InstrumentInfo",
+    "Interval",
+    "MoomooClient",
+    "create_moomoo_client",
+    "RateLimiter",
+    "CircuitBreaker"
+]