KIP-788 — Allow configuring num.network.threads per listener

Accepted Kafka 3.2 Broker

Allows `num.network.threads` to be configured per listener via the `listener.name.<NAME>.num.network.threads` notation so each listener's thread pool can be sized independently. A single global `num.network.threads` value is wasteful when listeners carry very different traffic volumes (e.g. inter-broker vs. external client listeners).

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