How does the KIP-848 consumer rebalance protocol differ from the classic protocol, and how does a consumer opt in?

In the classic protocol every membership change sends the whole group through a synchronization barrier: members revoke their partitions, rejoin, and wait for an assignment computed on the client side by the group leader. Even cooperative rebalancing depends on that barrier. KIP-848 moves the work to the broker: the group coordinator computes a target assignment with a server-side assignor (uniform or range) and each member reconciles toward it incrementally through its heartbeats; the design goal is that a member whose assignment does not change is not impacted. It is generally available in 4.0 and a client opts in with group.protocol=consumer (the default is classic). Session timeout and heartbeat interval become broker settings (group.consumer.session.timeout.ms and group.consumer.heartbeat.interval.ms), and client-side assignors are not supported.

Expected key concepts: KIP-848, group.protocol, consumer, coordinator, server-side assignor, heartbeat, barrier, incremental, uniform, range, reconcile