Support creation of DistributionPackages for delete by DistributionContentSerializer

For the integration into systems or then sling, that support creation/deletion as part of their API [1], we can implement a DistributionContentSerializer writing the format the API expects. That currently works well for creation but not for deletion because both FileDistributionPackageBuilder and ResourceDistributionPackageBuilder inherit from AbstractDistributionPackageBuilder which returns a SimpleDistributionPackage for DistributionRequestType.DELETE.

The other system's API might expect a different format then the one written by SimpleDistributionPackage so it would be create if it would be possible to delegate the creation of a deletion package to the serializer as well, in case the serializer supports that.

[1] https://lucene.apache.org/solr/guide/7_2/uploading-data-with-index-handlers.html