KIP-445 — In-memory Session Store

Accepted Kafka 2.3 Streams

Adds an in-memory `SessionStore` implementation to Kafka Streams via a new `Stores.inMemorySessionStore()` factory method. The session store type previously lacked an in-memory variant, forcing all session-windowed aggregations to use RocksDB even for workloads that fit in heap and do not need persistence.

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