Add duplicate() function to Query Builder

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

src/Functions/Duplicate.php

@@ -0,0 +1,35 @@
+<?php
+
+namespace Arendsen\FluxQueryBuilder\Functions;
+
+use Arendsen\FluxQueryBuilder\Type;
+use Arendsen\FluxQueryBuilder\Type\ArrayType;
+
+class Duplicate extends Base
+{
+    /**
+     * @var string $column
+     */
+    private $column;
+
+    /**
+     * @var string $as
+     */
+    private $as;
+
+    public function __construct(string $column, string $as)
+    {
+        $this->column = $column;
+        $this->as = $as;
+    }
+
+    public function __toString()
+    {
+        $input = new ArrayType([
+            'column' => $this->column,
+            'as' => $this->as
+        ]);
+
+        return '|> duplicate(' . $input . ') ';
+    }
+}

src/QueryBuilder.php

@@ -6,6 +6,7 @@ use DateTime;
 use Exception;
 use Arendsen\FluxQueryBuilder\Expression\KeyValue;
 use Arendsen\FluxQueryBuilder\Functions\AggregateWindow;
+use Arendsen\FluxQueryBuilder\Functions\Duplicate;
 use Arendsen\FluxQueryBuilder\Functions\Filter;
 use Arendsen\FluxQueryBuilder\Functions\From;
 use Arendsen\FluxQueryBuilder\Functions\Range;
@@ -29,6 +30,7 @@ class QueryBuilder
     public const FLUX_PART_LIMIT = 'limit';
     public const FLUX_PART_WINDOW = 'window';
     public const FLUX_PART_MEAN = 'mean';
+    public const FLUX_PART_DUPLICATE = 'duplicate';
     public const FLUX_PART_UNWINDOW = 'unwindow';
     public const FLUX_PART_AGGREGATEWINDOW = 'aggregateWindow';
 
@@ -38,6 +40,7 @@ class QueryBuilder
         self::FLUX_PART_REDUCE,
         self::FLUX_PART_WINDOW,
         self::FLUX_PART_MEAN,
+        self::FLUX_PART_DUPLICATE,
         self::FLUX_PART_FILTERS,
         self::FLUX_PART_MAP,
         self::FLUX_PART_SORT,
@@ -187,7 +190,16 @@ class QueryBuilder
         return $this;
     }
 
-    public function addMean(?string $column = '')
+    public function addDuplicate(string $column, string $as): QueryBuilder
+    {
+        $this->addToQuery(
+            self::FLUX_PART_DUPLICATE,
+            new Duplicate($column, $as)
+        );
+        return $this;
+    }
+
+    public function addMean(?string $column = ''): QueryBuilder
     {
         $this->addToQuery(
             self::FLUX_PART_MEAN,
@@ -196,7 +208,7 @@ class QueryBuilder
         return $this;
     }
 
-    public function addUnWindow()
+    public function addUnWindow(): QueryBuilder
     {
         $this->addToQuery(
             self::FLUX_PART_UNWINDOW,