KIP-557 — Add emit on change support for Kafka Streams

Accepted Kafka 2.6 Streams

Adds an `emit-on-change` processing mode to Kafka Streams aggregations so that downstream records are only forwarded when the aggregated value actually changes, suppressing redundant identical updates. Currently, Kafka Streams only supports `emit-on-update` (always forward) and `emit-on-window-close` (suppress until window closes), with no way to deduplicate identical successive aggregation results.

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