Kafka producer action's `topic` config now supports templates.

The topics must be already created in Kafka. If a message is rendered towards a non-existing topic in Kafka (given Kafka disabled topic auto-creation), the message will fail with an unrecoverable error.  Also, if a message does not contain enough information to render to the configured template (e.g.: the template is `t-${t}` and the message context does not define `t`), this message will also fail with an unrecoverable error. 

This same feature is also available for Azure Event Hubs and Confluent Platform producer integrations.