Integrate CustomMarshaller on model defining modules

[fjtirado]

CustomObjectMarhsaller class is defined on serverlessworkflow-persistence-api.
This implies that CustomObjectMarshaller implementations linked to a particular WorkflowModel has to be defined in a separate module, for example serverlessworkflow-persistence-jackson-marshaller
It will be better if marshaller are defined within the own model module, so, when users enable persistence they do not have to add the model marshaller module.
To do so, CustomObjectMarshaller should be moved to core.
Although we consider keeping the existing marshaller module for backward compatibility, we decided it is better to remove it.
This issue will also implement default marshaller for experimental Java model