Adds integrated support for the roundtripping of data associated with graph elements.

Adds integrated support for the roundtripping of data associated with graph elements. Calling this method after the creation of this instance will prepare the io mechanism to read and write the associated data from the requests and to the responses. Also the corresponding IMapper will be automatically registered with the graph's IMapperRegistry using the provided key on the graph instances created by CreateRoundtripGraph()()()()