KIP-593 — Enable --if-exists and --if-not-exists for AdminClient in TopicCommand

Discarded Admin

Extends `kafka-topics.sh` to support `--if-exists` and `--if-not-exists` flags when using `--bootstrap-server`, bringing the AdminClient-backed code path to parity with the ZooKeeper-backed path. Using `--bootstrap-server` with these idempotency flags previously resulted in an error because only the ZooKeeper code path handled them.

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