- Create INetworkManager interface for network abstraction
- Add EthernetManager for ENC28J60 module using EthernetENC library
- Update WiFiManager to implement INetworkManager interface
- Update MQTTBridge to use INetworkManager* instead of WiFiManager&
- Add heltec_v3_ethernet PlatformIO environment
- Uses HSPI bus (GPIO 19/20/47/48) separate from LoRa SPI