Configure spring-kafka to disable JSON encoding for specific headers

🎁 Enhancement

I am trying to integrate spring-kafka with opentracing-contrib/java-kafka-client. It defines an interceptor for Kafka ProducerRecords and expect to have uber-trace-id as a String. I would like to be able to configure the default KafkaHeaderMapper to ignore conversion of some specific header to JSON like the 'uber-trace-id'.
