Add ability to define graph of resources returned in topic.

Options include

1) Extension on subscription which references either a List or Graph of resource that the server will send (content_type = id_only or full resources) in any channel. IF the server supports and advertises its support.

2) Server to advertise unique topic for each graph of resources it supports. In other words topic1 - send Encounter, topic1a - send Encounter,Patient, Condition.  The subscriber then chooses the topic based on it needs.

Graphs could be defined by implementation guides and server conforming to those guides would need to offer the topic + graph.