KIP-313 — Add KStream.flatTransform and KStream.flatTransformValues

Accepted Kafka 2.2 Streams

KIP-313 adds `KStream.flatTransform()` and `KStream.flatTransformValues()` operators to the Kafka Streams DSL, enabling stateful one-to-many record transformations. The existing `KStream.transform()` only supports one-to-one (or zero-to-one) transformations via `Transformer`, requiring users to drop to the lower-level Processor API for stateful fan-out operations.

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