KIP-645 — Replace Windows with a proper interface

Discarded Streams

Refactors `Windows` from an abstract class to an interface in the Kafka Streams DSL so custom window types can be implemented without inheriting from a class that imposes opinionated fields. The abstract class forced concrete implementations to carry fields like `maintainDurationMs` and `gracePeriodMs` that don't apply to all window semantics, and blocked composition with other interfaces.

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