added annotation based exporter (#1831)

* added user exporter
* added project exporter
* added customer exporter
* added activity exporter

src/Export/Spreadsheet/Extractor/ExtractorInterface.php

@@ -0,0 +1,25 @@
+<?php
+
+/*
+ * This file is part of the Kimai time-tracking app.
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+namespace App\Export\Spreadsheet\Extractor;
+
+use App\Export\Spreadsheet\ColumnDefinition;
+
+/**
+ * Extract ColumnDefinition objects from various sources.
+ */
+interface ExtractorInterface
+{
+    /**
+     * @param mixed $value
+     * @return ColumnDefinition[]
+     * @throws ExtractorException
+     */
+    public function extract($value): array;
+}