KIP-88 / KAFKA-3853 Report offsets stored in Kafka for groups without an active consumer

Adding support for this would make writing external monitoring scripts much easier.

https://cwiki.apache.org/confluence/display/KAFKA/KIP-88%3A+OffsetFetch+Protocol+Update

https://issues.apache.org/jira/browse/KAFKA-3853

Structs for this were added in #971.

Related: #633