KIP-1300 — Update KStream-KStream Join to Use Plain-Value-Header Window Stores

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Introduces a `WindowStoreWithHeaders` and a `ValueAndHeaders` value wrapper that serialize window-store values as `[headersSize(varint)][headersBytes][value]`, and rewires the KStream-KStream join onto it instead of `TimestampedWindowStoreWithHeaders`. The join already reads the record timestamp from the window-store key and never reads the timestamp embedded in the stored value, so the current format wastes 8 bytes per record — roughly 7.3% of the store for 100-byte values in high-throughput join workloads.

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