Allow filtering or sending different event to different topics in the publisher.

[gklijs]

Feature Description

In some cases we want to only send certain events via Kafka. Also a typical Kafka setup is using a different topic for each message type. It would be nice if this was configurable.

Current Behaviour

Currently there is just one topic to which all events are sent.

Wanted Behaviour

Add an optional mapping lambda, from EventMessage to Optional KafkaPublisher<String, byte[]>. By default this will send all events to the default "Axon.Events" topic, if a topic is used in the builder, it will use a lambda that always returns the set topic.
If the lambda returns an empty optional the event will be skipped. Note that by using the lamda on the EventMessage, it's both versatile, and we can skip the conversion step if the event doesn't need to be sent via Kafka.
Something like:

m -> {
   if (m.getPayloadType() instance of WantedEvent) {
    return Optional.of("wanted-events-topic");
  } else {
    return Optional.empty()
  }
}

Possible Workarounds

Skipping the extension, and doing it yourself, for example with Spring Cloud Streams, which needs a lot more code to setup.