KIP-535 — Allow state stores to serve stale reads during rebalance

Discussion Kafka 2.6 Streams

Adds a `stale.metadata.allowed` configuration for Kafka Streams Interactive Queries that allows standby replicas to serve reads during an active rebalance, returning potentially stale state rather than throwing `InvalidStateStoreException`. During rebalances, all IQ requests fail because active tasks are being migrated, causing complete read unavailability even though standbys hold a recent copy of the state.

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