[FHIRcast] Add priorVersionId to DiagnosticReport-update on the Hub-distributed version

[ThatOneBro]

"The Hub SHALL distribute a corresponding event to all Subscribers. The Hub SHALL replace the context.versionId in the request with a new context.versionId generated and retained by the Hub. The prior version, context.priorVersionId of the context is also provided to ensure that a Subscriber is currently in sync with the latest context prior to applying the new updates. If the value of context.priorVersionId is not in agreement with the context.versionId last received by a Subscriber, it is recommended that the Subscriber issue a GET request to the Hub in order to retrieve the latest version of the context (note that the GET request returns the context, all existing content, and the current context.versionId)."

Source: https://build.fhir.org/ig/HL7/fhircast-docs/3-6-3-DiagnosticReport-update.html#diagnosticreport-update-event-example