[FEATURE] Support pushdown for subaggregation in PPL #4762
Description
Is your feature request related to a problem?
With #3550, we started single group-by aggregation pushdown, benefiting from upto 100x performance improvement for numeric aggregations. As part of opensearch-project/OpenSearch#19573, we are working significantly speeding up the execution even in the case of sub aggregation.
What solution would you like?
Multiple group-by aggregation pushdown to benefit from the optimizations in OpenSearch core search execution engine