KIP-614 — Add Prefix Scan support for State Stores

Accepted Kafka 2.8 Streams

Adds a `prefixScan(prefix, serializer)` method to `ReadOnlyKeyValueStore` and its RocksDB/in-memory implementations in Kafka Streams for efficiently retrieving all keys sharing a common byte prefix. Currently there is no native prefix query; developers must use `range()` with a manually computed upper bound or iterate the full key space, incurring unnecessary I/O.

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