Add reduce function to QueryBuilder

Signed-off-by: davidarendsen <davidarendsen@hey.com>

tests/Function/ReduceFunctionTest.php

@@ -0,0 +1,18 @@
+<?php
+declare(strict_types=1);
+
+use Arendsen\FluxQueryBuilder\Function\Reduce;
+use PHPUnit\Framework\TestCase;
+
+final class ReduceFunctionTest extends TestCase {
+
+    public function testSimpleReduce()
+    {
+        $expression = new Reduce(['sum' => 'r._value + accumulator.sum'], ['sum' => 0]);
+
+        $query = '|> reduce(fn: (r, accumulator) => ({sum: r._value + accumulator.sum}), identity: {sum: 0}) ';
+
+        $this->assertEquals($query, $expression->__toString());
+    }
+
+}