KIP-420 — Add Single Value Fetch in Session Stores

Accepted Kafka 2.3 Streams

Adds a `fetch(Bytes key, long startTime, long endTime)` single-point lookup method to the `SessionStore` interface, enabling efficient O(log N) retrieval of a specific session window by key and exact time bounds. Without single-value fetch, retrieving a specific session required a range scan over all sessions for a key, even when the exact start and end time of the target session are known.

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