Replacing full uri in ServiceMetadataReference with document type identifier

From https://lists.oasis-open.org/archives/bdxr-comment/201611/msg00004.html

I would prefer, if the ServiceMetadataReference would not contain the full target URL with participant and document type, but only the document type. The final URL can easily be build by SMP clients and it would be much easier to achieve this. Especially if an SMP is running behind a proxy, the SMP must always know it's "public URL" to be used in this element. This leads to problems in production, if e.g. the URLs between the ServiceGroup query and the contained ServiceMetadataReference are different (seen this quite often!)