Support 'group.protocol' for KafkaConsumer

[alexeymarunin]

Description

Is it possible to use a new rebalance protocol?

https://www.confluent.io/blog/kip-848-consumer-rebalance-protocol/
https://docs.confluent.io/platform/current/installation/configuration/consumer-configs.html#group-protocol

The following code:

<?php
$conf = new \RdKafka\Conf();
// ...
$conf->set('group.protocol', 'consumer');
// ...
$consumer = new \RdKafka\KafkaConsumer($conf);

Resulted in this output:

No such configuration property: "group.protocol"