Kafka: Add a feature to include headers in the messages

[mariaefi29]

At the current implementation schaufel does not process headers in Kafka messages.

Due to that problem we are not able to instrument proper tracing in services, which consume messages from schaufel.

For example, if we pass tracing information via headers in a message in a queue, from which schaufel consumes:

traceparent=00-trace_id-span_id-0

another service, which consumes from the queue, to which schaufel produce messages, will not get this tracing information, because headers are empty. Instead this service has only orphan spans and is not connected to the needed trace.

We would like to add the following feature in schaufel:

• if the incoming message contains headers, schaufel must pass those headers further without any modification.