KIP-810 — Allow producing records with null values in Kafka Console Producer

Accepted Kafka 3.2 ProducerAdmin

Adds a `null.marker` property to `kafka-console-producer.sh` that, when set, treats any matching input string as a null value, enabling production of tombstone records from the CLI. There is currently no way to produce null-valued records (tombstones) via the console producer, requiring users to rely on external tools like `kcat`.

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