KIP-149 — Enabling key access in ValueTransformer, ValueMapper, and ValueJoiner

Accepted Kafka 1.1 Streams

Extends `ValueTransformer`, `ValueMapper`, and `ValueJoiner` interfaces in Kafka Streams to expose the associated record key as a read-only parameter alongside the value. Without key access, developers must use the heavier `Transformer`/`KeyValueMapper` interfaces even when only inspecting the key, producing unnecessary `KeyValue` object allocations.

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