Provide default implementation for RowFunction evalDimension method (#15452)

The PR: #13947 introduced a function evalDimension() in the interface RowFunction.
There was no default implementation added for this interface which causes all the implementations and custom transforms to fail and require to implement their own version of evalDimension method. This PR adds a default implementation in the interface which allows the evalDimension to return value as a Singleton array of eval result.

processing/src/main/java/org/apache/druid/segment/transform/RowFunction.java

@@ -20,6 +20,7 @@
 package org.apache.druid.segment.transform;
 
 import org.apache.druid.data.input.Row;
+import org.apache.druid.data.input.Rows;
 
 import java.util.List;
 
@@ -30,5 +31,8 @@ public interface RowFunction
 {
   Object eval(Row row);
 
-  List<String> evalDimension(Row row);
+  default List<String> evalDimension(Row row)
+  {
+    return Rows.objectToStrings(eval(row));
+  }
 }

processing/src/test/java/org/apache/druid/segment/transform/RowFunctionTest.java

@@ -0,0 +1,84 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+
+package org.apache.druid.segment.transform;
+
+import org.apache.druid.data.input.Row;
+import org.apache.druid.data.input.Rows;
+import org.joda.time.DateTime;
+import org.junit.Assert;
+import org.junit.Test;
+
+import javax.annotation.Nullable;
+import java.util.List;
+
+public class RowFunctionTest implements RowFunction
+{
+  @Override
+  public Object eval(Row row)
+  {
+    return row;
+  }
+
+  @Test
+  public void defaultEvalDimensionTest()
+  {
+    Row row = new Row()
+    {
+      @Override
+      public long getTimestampFromEpoch()
+      {
+        return 0;
+      }
+
+      @Override
+      public DateTime getTimestamp()
+      {
+        return null;
+      }
+
+      @Override
+      public List<String> getDimension(String dimension)
+      {
+        return null;
+      }
+
+      @Nullable
+      @Override
+      public Object getRaw(String dimension)
+      {
+        return dimension;
+      }
+
+      @Nullable
+      @Override
+      public Number getMetric(String metric)
+      {
+        return null;
+      }
+
+      @Override
+      public int compareTo(Row o)
+      {
+        return 0;
+      }
+    };
+    Assert.assertEquals(Rows.objectToStrings(eval(row)), evalDimension(row));
+  }
+}