IE-100: Kafka:o-c-a: topic and message-key attrs

JIRA: https://jira.spring.io/browse/INTEXT-100

Add `topic(-expression)` and `message-key(-expression)` attributes to avoid upstream configuration
to specify them in the `MessageHeaders`

Polishing for XSD
A bit late now, but these should really be `KafkaHeaders` constants with prefixes (`kafka_*`).