Add sum() Flux function + docs

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

src/Functions/Sum.php

@@ -0,0 +1,21 @@
+<?php
+
+namespace Arendsen\FluxQueryBuilder\Functions;
+
+class Sum extends Base
+{
+    /**
+     * @var string $column
+     */
+    private $column;
+
+    public function __construct(string $column)
+    {
+        $this->column = $column;
+    }
+
+    public function __toString()
+    {
+        return '|> sum(column: "' . (string)$this->column . '") ';
+    }
+}