FILTER Clause for Aggregate Functions

[yjshen]

Is your feature request related to a problem or challenge?

Implement the FILTER clause for aggregate functions (introduced in SQL:2003). This syntax allows for filtering before aggregation.

SUM(<expression>) FILTER(WHERE <condition>)

Describe the solution you'd like

Have the filter support directly in our AggregateExec.

Describe alternatives you've considered

Rewrite the filter clause for aggregate functions to use the CASE WHEN expression during planning. However, null handling for each aggregator function regarding the filter would be error-prone.

Additional context

PostgreSQL: https://www.postgresql.org/docs/current/sql-expressions.html#SYNTAX-AGGREGATES
Spark: https://spark.apache.org/docs/3.3.2/sql-ref-syntax-qry-select-groupby.html