Event-specific payload. Shape is determined by message_type; see
the developer portal for the per-event schema. For
proxy_key.registration_completed the payload follows
simulator.proxy_key.registration_completed.payload; for
proxy_key.registration_rejected it follows
simulator.proxy_key.registration_rejected.payload; for
proxy_key.invalidated_by_scheme it follows
simulator.proxy_key.invalidated_by_scheme.payload; for
proxy_claim.creation_completed it follows
simulator.proxy_claim.creation_completed.payload; for
proxy_claim.creation_rejected it follows
simulator.proxy_claim.creation_rejected.payload; for
proxy_claim.acknowledged_by_donor it follows
simulator.proxy_claim.acknowledged_by_donor.payload; for
proxy_claim.confirmed_by_donor it follows
simulator.proxy_claim.confirmed_by_donor.payload; for
proxy_claim.cancelled_by_donor it follows
simulator.proxy_claim.cancelled_by_donor.payload; for
proxy_claim.completion_completed it follows
simulator.proxy_claim.completion_completed.payload; for
proxy_claim.completion_rejected it follows
simulator.proxy_claim.completion_rejected.payload; for
proxy_claim.cancellation_completed it follows
simulator.proxy_claim.cancellation_completed.payload; for
proxy_claim.cancellation_rejected it follows
simulator.proxy_claim.cancellation_rejected.payload; for
proxy_claim.received it follows
simulator.proxy_claim.received.payload; for
proxy_claim.acknowledgment_completed it follows
simulator.proxy_claim.acknowledgment_completed.payload; for
proxy_claim.acknowledgment_rejected it follows
simulator.proxy_claim.acknowledgment_rejected.payload; for
proxy_claim.confirmation_completed it follows
simulator.proxy_claim.confirmation_completed.payload; for
proxy_claim.confirmation_rejected it follows
simulator.proxy_claim.confirmation_rejected.payload; for
proxy_claim.completed_by_claimer it follows
simulator.proxy_claim.completed_by_claimer.payload; for
proxy_claim.cancelled_by_claimer it follows
simulator.proxy_claim.cancelled_by_claimer.payload; for
proxy_claim.auto_confirmed it follows
simulator.proxy_claim.auto_confirmed.payload.