KIP-450 — Sliding Window Aggregations in the DSL

Accepted Kafka 2.7 Streams

Adds a `SlidingWindows` window type to the Kafka Streams DSL, inclusive on both start and end time, exposed through new `windowedBy(SlidingWindows)` overloads on `KGroupedStream` and `CogroupedKStream`. Emulating sliding aggregation with hopping windows advanced by 1 ms produces many overlapping and often redundant windows whose aggregates are recomputed each time, whereas sliding windows compute each distinct window exactly once.

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