Feat: support stddev and variance function

Author: taewhi

traindb-core/src/main/java/traindb/planner/rules/ApproxAggregateUtil.java

@@ -33,13 +33,15 @@ public class ApproxAggregateUtil {
 
  private static final List<String> approxAggregateFuncList = Arrays.asList(
  "AVG",
+ "COUNT",
+ "STDDEV",
  "SUM",
- "COUNT"
+ "VARIANCE"
  );
 
  private static final List<String> scalingAggregateFuncList = Arrays.asList(
- "SUM",
- "COUNT"
+ "COUNT",
+ "SUM"
  );
 
  public static boolean isApproximateAggregate(Aggregate aggregate) {