Expose StreamController(s) (MeshPacketWrapper) for allowing to proxy MQTT messages into the stream

[eelco2k]

In my app I want to receive and send meshtastic data through MQTT broker. which the official meshtastic app also offers.

So when I receive an mqtt message which corresponds to the correct topic msh/REGION/2/e/CHANNELNAME/USERID i want to inject it into the packageStream.
but because _packetController is private i cannot .add() to the stream...

meshtastic_client.dart

final StreamController<MeshPacketWrapper> _packetController =
      StreamController<MeshPacketWrapper>.broadcast();

so maybe just add something like:

  StreamController<MeshPacketWrapper> get packetController => _packetController;

[eelco2k]

#3