Add mean() method to Query Builder

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

tests/Functions/MeanFunctionTest.php

@@ -0,0 +1,29 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Tests\Functions;
+
+use Arendsen\FluxQueryBuilder\Functions\Mean;
+use PHPUnit\Framework\TestCase;
+
+final class MeanFunctionTest extends TestCase
+{
+    public function testSimpleMean()
+    {
+        $expression = new Mean();
+
+        $query = '|> mean() ';
+
+        $this->assertEquals($query, $expression->__toString());
+    }
+
+    public function testWithParameterColumn()
+    {
+        $expression = new Mean('test');
+
+        $query = '|> mean(column: "test") ';
+
+        $this->assertEquals($query, $expression->__toString());
+    }
+}