Support groovy UDF transform in post-aggregation

Currently only scalar UDF is supported for post-aggregation transform, when user want to transform an aggregation result, user need to create a scalar UDF for every case.

Pinot need to support groovy UDF for post-aggregation transform.

Start by locating Pinot's post-aggregation transform handling and the existing scalar UDF implementation. Inspect related aggregation or transform tests, then verify that a Groovy UDF can transform aggregation results without requiring a separate scalar UDF for each case.

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