KIP-336 — Consolidate ExtendedSerializer/Serializer and ExtendedDeserializer/Deserializer

Accepted Kafka 2.1 Client

Merges ExtendedSerializer into Serializer and ExtendedDeserializer into Deserializer, adding the header-aware serialize/deserialize methods directly to the base interfaces. KIP-82 introduced ExtendedSerializer/Deserializer as separate interfaces to avoid breaking compatibility, but the split created confusion and required instanceof checks throughout the codebase; with Java 8 minimum, default methods allow a clean merge.

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