[FEATURE] Pushdown any UDAF by scripted metric aggregations

[songkant-aws]

Is your feature request related to a problem?
Currently, complex UDAFs like patterns command implement its own logic because OpenSearch core doesn't provide the corresponding aggregator implementation. For those UDAFs, it's not straightforward to pushdown aggregation by composite aggregation for them.

OpenSearch scripted metric aggregation provides a pipeline of multiple mapping to single reduce by multiple scripts. See: https://docs.opensearch.org/latest/aggregations/metric/scripted-metric/

DataNode1: initScript -> mapScript -> combineScript
                                                     \
                                                      \
DataNode2: initScript -> mapScript -> combineScript -> reduceScript -> agg result
                                                      /
                                                     /
DataNode3: initScript -> mapScript -> combineScript

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[songkant-aws]

I can pick this one