KIP-765 — Introduce new SlidingWindow type for [start,end] time

Discussion Streams

Introduces a new `SlidingWindow` class with a closed `[start, end]` time interval (inclusive on both sides) to correctly represent the semantics of sliding window aggregations in Kafka Streams. The existing `TimeWindow` uses a half-open `[start, end)` interval, which misrepresents the intended semantics of sliding windows where the end time should be inclusive.

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