Add possibility to specify key fields in kafka producer

[oscni]

Is your feature request related to a problem? Please describe. Does not seem to be possible to send messages to kafka topic with a key specified. Key currently reports as Null

Describe the solution you'd like Possibility to specify key fields in kafka producer, maybe the same way as for filtering allow list

Describe alternatives you've considered None

Additional context Use JSON format and make it possible to specify multiple data fields as key