Introduce grouping in TopN operator

Author: przemekwitek

x-pack/plugin/esql/compute/src/main/java/org/elasticsearch/compute/data/Page.java

@@ -16,6 +16,7 @@
 import java.io.IOException;
 import java.util.Arrays;
 import java.util.Objects;
+import java.util.stream.IntStream;
 
 /**
  * A page is a column-oriented data abstraction that allows data to be passed between operators in
@@ -311,4 +312,9 @@ public Page filter(int... positions) {
         }
         return new Page(filteredBlocks);
     }
+
+    public Page subPage(int fromIndex, int toIndex) {
+        // TODO: optimize!
+        return filter(IntStream.range(fromIndex, toIndex).toArray());
+    }
 }