Support {{topic}} variable with {{key}} in file template pattern

[stephen-harris]

This is a feature request to support use of {{key}} in conjunction with {{topic}} variable.

Currently two modes of grouping are supported:

• grouping by the topic, partition, and timestamp;
• grouping by the key.

In our context we have a connector responsible for backing-up multiple compacted topics to an S3 bucket. While this could be achieved through multiple connectors, but given the large number of topics we were hoping to use connectors to provide a logical grouping of topics. However, as keys may exist across topics, to do this we would need the topic name in the S3 key.