KIP-924 — customizable task assignment for Streams

Accepted Kafka 3.8 Streams

KIP-924 promotes the `task.assignor.class` configuration in Kafka Streams to a public API, allowing users to select between `HighAvailabilityTaskAssignor` and `StickyTaskAssignor` (or supply a custom implementation). KIP-441 introduced `HighAvailabilityTaskAssignor` as the new default but the existing `StickyTaskAssignor` remains preferable in some use cases, and the existing backdoor config was internal with no stability guarantee.

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