* Refactored the creation of the ``JsonSerde``s, that are used to consume
the incomming messages.
* All special ``Serdes``, that are used for incomming and outgoing messages,
are created in separted methods now.
* Removed unnecessary operatorx in the ``Materialized``-configuration for
the state store (the operator is not necessary, because no headers are
present, when deserializing from a store).