Add convenience methods to ConsumerProducerBuilder

[jkbe]

I think in a follow up PR we might want to create some convenience methods on ConsumerProducerBuilder to directly access consumer and producer.

Perhaps we want to hide the subscribeToAllTopics in the builder as well by default. Usually, it should be fine to immediately subscribe to all topics after creating the consumer, right?

Originally posted by @jkbe in #387 (comment)