Implement a UDP networking service

[Levi-Lesches]

We need a RoverNetwork service that can:

• establish a connection with the rover (it's UDP, so probably just a few pings)
• send messages to the rover as serialized Protobuf messages
• receive Protobuf messages from the rover

Loosely going from the above, we'd probably need the following:

class RoverNetwork extends Service {
  Future<void> init();  // send pings until receive response
  Future<void> send(RoverMessage message);  // sends a message to the rover
  Stream<RoverMessage> incomingMessages;  // receives incoming messages from the rover
}

We'd use a stream for incoming messages since dart:io uses it for its RawDatagramSocket UDP implementation. It will also help the front end, which has built-in support for updating widget values based on data from a Stream object.

[superdan-t]

This is blocked by the draft issue Revise network message-passing protocol for inter-language implementation.

It might be worth putting in a separate repo with the other network stuff but we can decide that when the task is unblocked.

(Also bumped from med to large since I think this is one of the bigger tasks)

[Levi-Lesches]

We've decided to go with a Stream-like API, but without directly exposing streams. We're using:

class MessageSender extends Service {
  /// Wraps [message] in a [WrapperMessage] and sends over UDP.
  Future<void> send(Message message);   
}

class MessageReceiver extends Service {
  /// Runs [handler] with the message decoded by [decoder] whenever a message of type [name] appears.
  void registerHandler<T extends Message>({
    required String name, 
    required MessageDecoder<T> decoder, 
    required Handler<T> handler
  });
}