FHIRcast notification is not a FHIR resource

[wayfarer3130]

The FHIRcast notification should be a standard FHIR resource, both parsed and generated by the standard FHIR libraries as actual FHIR objects. Ideally, this should be shared with the Subscription in R5, but at a minimum it should be a real FHIR object. This type of object might not be fetchable from a FHIR server, but that sort of thing could be added to allow recording the set of events leading up to a FHIRcast setup, or to allow notification of just the id that changes to be used to recover the full FHIRcast event notification. It also means there are standard places to put extension points and a standard mechanism to define them.