Kafka producer headers not supported?

[SkatoriDev]

It seems that there is either no support for headers or I just can't figure out how to send them.
Underlying rd_kafka_producev does not utilize *_HEADER(S) macros
Can you please either provide an example of sending message with headers or tell about plans to add the support for them? If there are none in near future, I might make a pr

[fdr400]

Our internal services do not use headers, though there was no need for them yet.
We thought about their support, but not in near future.
So, we are glad to look at your PR!