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
I think we can do that right now, because we don't have yet any GA, so end-users should admit that our next Milestone may change something radically