Add addUnique function

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

tests/Functions/UniqueFunctionTest.php

@@ -0,0 +1,29 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Tests\Functions;
+
+use Arendsen\FluxQueryBuilder\Functions\Unique;
+use PHPUnit\Framework\TestCase;
+
+final class UniqueunctionTest extends TestCase
+{
+    public function testSimpleUnique()
+    {
+        $expression = new Unique();
+
+        $query = '|> unique() ';
+
+        $this->assertEquals($query, $expression->__toString());
+    }
+
+    public function testUniqueWithColumn()
+    {
+        $expression = new Unique('something');
+
+        $query = '|> unique(column: "something") ';
+
+        $this->assertEquals($query, $expression->__toString());
+    }
+}