Add mean() method to Query Builder

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

src/Functions/Mean.php

@@ -0,0 +1,29 @@
+<?php
+
+namespace Arendsen\FluxQueryBuilder\Functions;
+
+use Arendsen\FluxQueryBuilder\Type;
+use Arendsen\FluxQueryBuilder\Type\ArrayType;
+
+class Mean extends Base
+{
+    /**
+     * @var string $column
+     */
+    private $column;
+
+    public function __construct(string $column = '_value')
+    {
+        $this->column = $column;
+    }
+
+    public function __toString()
+    {
+        $input = new ArrayType(array_filter([
+            'column' => !empty($this->column) && $this->column !== '_value'  ?
+                new Type($this->column) : null
+        ]));
+
+        return '|> mean(' . $input . ') ';
+    }
+}