KIP-728 — Introduce the RESTORING state to the KafkaStreams state machine

Discussion Streams

Introduces a new `RESTORING` state to the `KafkaStreams` state machine that sits between `REBALANCING` and `RUNNING`, remaining active until all state stores have completed changelog restoration. Without a distinct `RESTORING` state, applications and monitoring systems cannot distinguish between a cluster actively rebalancing and one that is catching up on state store restoration after a rebalance.

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