Related to currently resolved issue: https://jira.hl7.org/browse/FHIR-37280

STU2 did not explicitly establish a requirement for all content for FHIRcast events to be in the form of FHIR resources.  For events defined in the Event Catalog, such a requirement is reasonable, given the established definitions specify using FHIR resources.  Introducing this requirement for custom events, which are not included in the published event catalog, may break backwards compatibility for existing systems and introduce an additional step of processing for participating systems to translate simple data to/from FHIR resources.  I propose we should allow for non-FHIR context, such as simple key/value context, in custom events (those defined using reverse-domain notation).