Expose a way to set offset manager config

[aaronkavlie-wf]

While there are public methods for initializing the offset manager and its config, JoinConsumerGroup() does not make it possible to set this. It sets internally to a private member, offsetManager. I don't see a way to set VerboseLogging in the consumer group API.

[wvanbergen]

How would you want to initialize it?

[aaronkavlie-wf]

Seems that an OffsetManagerConfig pointer would be at home in the Config struct in consumer_group.go, similar to the *kazoo.Config pointer.