KIP-622 — Add currentSystemTimeMs and currentStreamTimeMs to ProcessorContext

Accepted Kafka 3.0 StreamsTesting

Exposes `currentSystemTimeMs()` and `currentStreamTimeMs()` methods on `ProcessorContext` in Kafka Streams, sourcing the timestamps from the Streams runtime rather than `System.currentTimeMillis()`. This makes wall-clock and stream-time usage testable because `MockTime` can control both values during unit tests.

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