Akka actor for publishing messages to Kafka

[cmontemuino]

We need to implement an Akka actor that publish messages to Kafka in Akka Stream context.

In order to make this work, it is expected to create a producer manager that instructs a producer actor to publish messages into Kafka.

As a starting point, we can simplify the publishing scenario and write String-based messages, and use no Key. We can add keyed-based messages, and use Avro for encoding in future tasks.

It is needed to create create a Main app in the demo module to show how the new created artifacts work.