Move preprocessing logic into Preprocessor class

[MAILPOET-2286]

Additional preprocessing logic is needed to handle WooCommerce
blocks, so instead of making the Renderer class even bigger,
I am creating a new Preprocessor class. I wanted to move the
`addMailpoetLogoContentBlock` logic as well but it will break
tests and require a lot of refactor to fix them so I am leaving
it here for now.

lib/Newsletter/Renderer/Preprocessor.php

@@ -0,0 +1,38 @@
+<?php
+
+namespace MailPoet\Newsletter\Renderer;
+
+use MailPoet\Newsletter\Renderer\Blocks\Renderer as BlocksRenderer;
+
+class Preprocessor {
+
+  /** @var BlocksRenderer */
+  private $blocks_renderer;
+
+  public function __construct(BlocksRenderer $blocks_renderer) {
+    $this->blocks_renderer = $blocks_renderer;
+  }
+
+  /**
+   * @param array $content
+   * @return array
+   */
+  public function process($content) {
+    if (!array_key_exists('blocks', $content)) {
+      return $content;
+    }
+    $blocks = [];
+    foreach ($content['blocks'] as $block) {
+      if ($block['type'] === 'automatedLatestContentLayout') {
+        $blocks = array_merge(
+          $blocks,
+          $this->blocks_renderer->automatedLatestContentTransformedPosts($block)
+        );
+      } else {
+        $blocks[] = $block;
+      }
+    }
+    $content['blocks'] = $blocks;
+    return $content;
+  }
+}