out_kafka: add support for dynamic/static headers

[DIFRIN]

Add support for dynamic or static header for the kafka output plugin

Example:
'docker run --rm --network kafka_default -e value_test='${text}' -it fluent/fluent-bit-local:latest -i dummy -p dummy='{"text": "hello world"}' -p samples=1 -o kafka -p brokers=kafka:9092 -p topics=test_fluentbit -p header="key2 <text>" -vvv'

Gives
kafka-console-consumer --topic test_fluentbit --bootstrap-server kafka:9092 --property print.headers=true
key2:hello world {"@timestamp":1710505560.027627,"text":"hello world"}

[lecaros]

Hi @DIFRIN,
please sort the DCO issue.

[DIFRIN]

Hi @lecaros

DCO ok. Can you please review my merge request at fluent-bit repo