Add visibility of keys for kafka records in KafkaSubscriber

[bkrahmer]

Background:
When using kafka topics which are 'compact', you hint to kafka that it can delete all records for a given key by publishing to the topic with a null body for the given key.

This data is available in the ConsumerRecord, I will add they key to the KafkaTopicInfo, since it is already passed to the event callback handler.

[bkrahmer]

Fixed in 3.0.17