KIP-513 — Distinguish between Key and Value serdes in scala wrapper library for kafka streams

Discussion Streams

KIP-513 proposes introducing `KeySerde[K]` and `ValueSerde[V]` wrapper types in the Kafka Streams Scala API so the compiler can statically distinguish serdes intended for keys (which must be configured with `isKey=true` for schema-registry serdes) from serdes intended for values. Without the distinction, code using the Confluent Schema Registry serde must manually manage the `isKey` flag, and mixing key and value serdes compiles successfully but fails at runtime.

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