Add private key export support

- Add PRIVATE_KEY and DISABLED event types
- Add packet parsing for private key export responses
- Add export_private_key() method to DeviceCommands
- Add comprehensive unit tests
- Add BLE private key export example
- Update documentation with security notes

src/meshcore/events.py

@@ -41,6 +41,8 @@ class EventType(Enum):
     CUSTOM_VARS = "custom_vars"
     CHANNEL_INFO = "channel_info"
     PATH_RESPONSE = "path_response"
+    PRIVATE_KEY = "private_key"
+    DISABLED = "disabled"
 
     # Command response types
     OK = "command_ok"