KIP-351 — Add --under-min-isr option to describe topics command

Accepted Kafka 2.3 AdminBroker

Adds an `--under-min-isr` flag to `kafka-topics.sh --describe` that filters the output to show only partitions whose ISR size is below `min.insync.replicas`. The existing `UnderMinIsrPartitionCount` JMX metric shows a count but not which partitions are affected, forcing operators to describe every topic and parse ISR sizes manually to identify the specific under-min-ISR partitions.

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