Support record headers

[0x2c7]

From Kafka 0.11.0, the new record batch format supports optional headers field for each message. This helps a lot in tracing, monitoring, as well as other stuff since it allows us to attach the metadata without messing up the main message content. The previous record batch implement PR already supports this field in record batch encoding / decoding. This PR is to implement the interfaces for the outsider to make use of this feature. The interfaces are available in Kafka::Client and Kafka::Producer and Kafka::AsyncProducer.

kafka.deliver_message(
   'hello', topic: 'world', headers: { 'TracingID' => 'a1234', 'SpanID' => '4567' }
)
producer = kafka.producer
producer.produce(
  'hello', topic: 'world', headers: { 'TracingID' => 'a1234', 'SpanID' => '4567' }
)

The consumer could access record headers via:

consumer.each_message do |message|
  puts message.headers
end

cc @mensfeld

[dasch]

❤️