No access to consumer in rebalance callbacks

[farico]

Currently we are limited to only receiving Rebalance enum on ConsumerContext which limits us from doing any offset manipulations (like committing offsets on partition that is about to be revoked) as it has no access to consumer:

impl ConsumerContext for CustomContext {
    fn pre_rebalance(&self, rebalance: &Rebalance) {
        info!("Pre rebalance {:?}", rebalance);
    }

    fn post_rebalance(&self, rebalance: &Rebalance) {
        info!("Post rebalance {:?}", rebalance);
    }

    fn commit_callback(&self, result: KafkaResult<()>, _offsets: &TopicPartitionList) {
        info!("Committing offsets: {:?}", result);
    }
}

This is rather limiting if we want to commit processed offsets during rebalance. Java API provides access to consumer and so does C++.

If we try to override the rebalance we still can't access NativeClient as it is private.

Can we also have access to consumer in pre_rebalance and post_rebalance?