[Feature] Support directory for KV inc cache

R2 uses a [directory](https://github.com/opennextjs/opennextjs-cloudflare/blob/31b31dd2dd2baad784fe2d29f01c059fa641dae4/packages/cloudflare/src/api/overrides/incremental-cache/r2-incremental-cache.ts#L81)

We should add it to KV for consistency

See discussion on https://github.com/opennextjs/opennextjs-cloudflare/pull/553