Add String Column Support for Count Distinct Aggregation (#1196)

* add string support for count distinct column

* fix: use native hashing algorithm and fix bug

feathr-impl/src/main/scala/com/linkedin/feathr/offline/swa/SlidingWindowFeatureUtils.scala

@@ -186,7 +186,9 @@ private[offline] object SlidingWindowFeatureUtils {
         // In Feathr's use case, we want to treat the count aggregation as simple count of non-null items.
         val rewrittenDef = s"CASE WHEN ${featureDef} IS NOT NULL THEN 1 ELSE 0 END"
         new CountAggregate(rewrittenDef)
-      case AggregationType.COUNT_DISTINCT => new CountDistinctAggregate(featureDef)
+      case AggregationType.COUNT_DISTINCT =>
+        val rewrittenDef = s"CASE WHEN ${featureDef} IS NOT NULL THEN hash(${featureDef}) ELSE 0 END"
+        new CountDistinctAggregate(rewrittenDef)
       case AggregationType.AVG => new AvgAggregate(featureDef)
       case AggregationType.MAX => new MaxAggregate(featureDef)
       case AggregationType.MIN => new MinAggregate(featureDef)