KIP-497 — Add inter-broker API to alter ISR

Accepted Kafka 2.7 ProtocolBroker

Introduces a new inter-broker `AlterIsrRequest`/`AlterIsrResponse` RPC that allows partition leaders to update the ISR via the controller over the Kafka protocol instead of writing directly to ZooKeeper. ZooKeeper-based ISR updates are a scalability bottleneck and a prerequisite for KIP-500's ZooKeeper-free architecture since the controller must be the single authoritative source for ISR state.

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