KIP-122 — Add Reset Consumer Group Offsets tooling

Accepted Kafka 1.0 ConsumerAdmin

Adds a `kafka-consumer-groups.sh --reset-offsets` subcommand that can reset consumer group offsets to earliest, latest, a specific offset, a specific datetime, or a relative shift, with `--dry-run` and `--execute` modes. Previously, offset resets required writing custom consumer code that called `KafkaConsumer#seek()`, redeploy the app, then roll it back — a slow and error-prone process.

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