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

.env.example

@@ -0,0 +1,13 @@
+# Moomoo API Configuration
+MOOMOO_API_KEY=your_api_key_here
+MOOMOO_SESSION_TOKEN=your_session_token_here
+
+# Redis Configuration (optional)
+REDIS_HOST=localhost
+REDIS_PORT=6379
+REDIS_PASSWORD=
+REDIS_DB=0
+
+# Application Settings
+DEBUG=True
+CACHE_TTL=300