* Make aggregation column block accessor private
Move ColumnBlockAccessor out of tantivy-columnar and into the aggregation implementation. Keep its behavior and tests intact while removing it from the public columnar API.
* Refactor aggregation value block access
* Following comment
Removing Fused from the element that are private to the module.
Re-added optimization in stats for single doc requests to avoid perf regression
Renamed Fused -> Flattened
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Co-authored-by: Paul Masurel <paul.masurel@datadoghq.com>