Send Content-Type header along with the DistributionPackage's content in forward and reverse distribution

Along with SLING-7357 the transmission of DistributionPackage's Content-Type has been implemented for forward distribution to allow writing DistributionContentSerializers targeting different kinds of sinks in forward distribution.

An addition to that use case is the implementation of the same in the opposite direction. For example sling not only being a source in a stream processing cluster but also being a sink. 

For example:

1. We have a authoring instance where editors create content
2. That content gets distributed to a publishing instance
3. From the publishing instance the content gets distributed to apache solr
4. Solr runs ClassificationUpdateProcessorFactory enriching the content
5. The authoring instance periodically pulls back the classified content

A real world example would be smart tagging, where human judgment is applied on a subset of content and classification with existing infrastructure is done to enrich the all of the content.