KIP-229 — DeleteGroups API

Accepted Kafka 1.1 ConsumerAdminProtocol

Adds a `DeleteGroups` admin API and corresponding `--delete` option to the `kafka-consumer-groups.sh` tool for explicitly deleting consumer groups backed by the new offset storage (`__consumer_offsets`). In the new consumer group management model, groups can only be removed when their committed offsets expire (default 1-day retention), leaving the offset store inflated for inactive groups with no explicit deletion path.

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