Add addUnique function

Signed-off-by: David Arendsen <darendsen@gamepoint.com>

src/Functions/Unique.php

@@ -0,0 +1,27 @@
+<?php
+
+namespace Arendsen\FluxQueryBuilder\Functions;
+
+use Arendsen\FluxQueryBuilder\Type;
+
+class Unique extends Base
+{
+    /**
+     * @var string|null $column
+     */
+    private $column;
+
+    public function __construct(?string $column = null)
+    {
+        $this->column = $column;
+    }
+
+    public function __toString()
+    {
+        $params = new Type(array_filter([
+          'column' => $this->column
+        ]));
+
+        return '|> unique(' . $params . ') ';
+    }
+}