Transport DistributionPackageInfo as Http headers

Currently DistributionPackages are sent over wire including there info as headers in the binary stream. In SLING-7357 I proposed that its the responsibility of the serializer to specify which kind of content it is serializing. Unfortunately as SimpleHttpDistributionTransport prepends the DistributionPackageInfo the content is not of the content type serialized anymore and the consumer side has to first read the package info before the actual content can be read.

For example in my demo to integrate SCD with solr I had to write a custom UpdateRequestHandler to be able to parse the json I'm sending to solr with SCD:

https://github.com/Buuhuu/sling-content-distribution-solr/blob/master/sling-content-distribution-solr-loader/src/main/java/com/github/buuhuu/solr/handler/SCDUpdateRequestHandler.java

If possible it would be nice to have those info transmitted using http headers, which would allow to integrate into other systems as they are.