examples: rdkafka example using message_record type which doesn't implement ToBytes trait

[cduggn]

Looking to add a fix or get clarification on how to use example at example-projects/rdkafka-example

   let message_record =
                MessageRecord::from_event(event).expect("error while serializing the event");

            let delivery_status = producer
                .send(
                    FutureRecord::to(topic_name)
                        .message_record(&message_record)
                        .key(&format!("Key {}", i)),
                    Duration::from_secs(10),
                )
                .await;

• Producer example is attempting to use MessageRecord to serialize events and send directly to Kafka.
• The FutureRecord types however requires a payload that implements ToBytes. MessageRecord doesn't have an impl for this
• It does have a payload field type Vec but that field is only visible to the rdkafka crate .

Adding a function for example pub fn payload and pub fn headerson MessageRecord would make the payload and header fields accessible