KIP-663 — API to Start and Shut Down Stream Threads

Accepted Kafka 2.8 Streams

Adds `KafkaStreams.addStreamThread()` and `KafkaStreams.removeStreamThread()` methods to allow dynamic scaling of stream thread count at runtime without restarting the Streams client. Currently, if a stream thread dies due to an uncaught exception, the only recovery option is to restart the entire `KafkaStreams` client, causing unnecessary downtime.

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