KIP-389 — Introduce a configurable consumer group size limit

Accepted Kafka 2.1.1 ConsumerBroker

Introduces a `group.max.size` broker configuration that enforces an upper bound on the number of members in a single consumer group, rejecting `JoinGroup` requests with `GROUP_MAX_SIZE_REACHED` when the limit is exceeded. Without a group size limit, arbitrarily large consumer groups can form, causing rebalance times and coordinator memory usage to grow unboundedly, degrading performance for all groups managed by that coordinator.

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