KIP-634 — Complementary support for headers and record metadata in Kafka Streams DSL

Discarded Streams

Extends the Kafka Streams DSL with `mapValueToRecord` and `setRecordHeaders` operators that expose record metadata (topic, partition, offset, timestamp, headers) inside stateless and stateful DSL transformations. Currently such access requires dropping down to the low-level Processor API and re-implementing DSL operations like joins from scratch.

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