Allow AsyncMessageBroker to recieve data from subscribers ?

[superkerokero]

AsyncMessageBroker seems to be useful in cases where one may need to collect some data from subscribers.

Currently, you can await all subscribers to finish their jobs, then do something like this:

• Publisher

  await AsyncMessageBroker.Default.PublishAsync<TestMessage>(new TestMessage());
  // Do something after

• Subscriber

  AsyncMessageBroker.Default.Subscribe<TestMessage>(_ =>
  {
  return Observable.TimerFrame(10).AsUnitObservable();
  }).AddTo(this);

But what I want to do is something like this:

• Publisher

  TestPayload[] payloads = await AsyncMessageBroker.Default.PublishAsync<TestPayload, TestMessage>(new TestMessage());
  // Do something with awaited payloads
  Debug.Log(payloads[0].Value);

• Subscriber

  AsyncMessageBroker.Default.Subscribe<TestMessage, TestPayload>(_ =>
  {
  return Observable.TimerFrame(10).Select(_ => new TestPayload(Value=1000));
  }).AddTo(this);

[superkerokero]

I have just submitted a PR #457 to address this issue.