Function handle_message 

async fn handle_message<Client>(
    message: &str,
    fhir_client: &Client,
    config: &ReceiverConfig,
) -> MessageOutcome
where
    Client: FHIRClient<(), OperationOutcomeError> + Send + Sync,

Converts one message and submits it.

Never returns an error: the caller owes the sender an acknowledgement in every case, so a failure is an [AckCode] rather than a ?.