KIP-478 — Strongly typed Processor API

Accepted Kafka 2.7 Streams

Adds compile-time type safety to the Kafka Streams Processor API by introducing typed `ProcessorSupplier<KIn, VIn, KOut, VOut>`, `FixedKeyProcessorSupplier`, and related interfaces so the processor chain's key-value types are checked at compile time. The original Processor API used raw types for inputs and outputs, allowing type mismatches to fail only at runtime.

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