Allow inclusion of "untyped JSON" into JSON requests and responses

Allow JSON payloads to contain "untyped JSON" in addition to OData declared and open properties. Add annotation for structured types that announce the potential presence of untyped JSON, either as a tagging term or by explicitly listing the names of the "untyped JSON" properties: <Annotation Term="Xxx.ContainsUntypedJSON" /> Or <Annotation Term="Xxx.ContainsUntypedJSON"> <Record> <PropertyValue Property="Properties"> <Collection> <String>opaqueJsonProperty</String> <String>Another-untyped/JSON;Property{with:a#weird name</String> </PropertyValue> </Collection> </Record> </Annotation> Allow clients to tell whether they are willing to receive "untyped JSON" via a new preference odata.allow-untyped-json.