Support for reading / writing kafka message headers

Hello,

Nice work on this project - it looks really cool!

My shop is looking at using this as the base for a golang stream processor. We have existing SPs which implement our message format / conventions, one of which is that we use kafka headers for message metadata (correlation_id, timestamp, ser/des details, etc).

I see that the provided `KafkaConsumer` passes on the received message's body to the next node, but does not pass headers.
- https://github.com/digitalocean/firebolt/blob/main/node/kafkaconsumer/kafkaconsumer.go#L221
- https://github.com/confluentinc/confluent-kafka-go/blob/master/kafka/message.go#L72-L80

Similarly, the provided `KafkaProducer` does not provide a way to assign/write headers (that I can see).
- https://github.com/digitalocean/firebolt/blob/main/node/kafkaproducer/kafkaproducer.go#L108-L111

Do you have any guidance on the best way to accomplish this?

One option seems to be to copy `KafkaConsumer` and `KafkaProducer` and modify them to do what we want. This seems inelegant, since there would be a ton of functionality in those that we would not be touching, but would be accepting a maintenance burden for.

Or maybe (hopefully?) I've missed something and there is some header functionality already available?

Thanks,
Dan