title: Add custom entity to email template in Shopware 6
body: <p>I'm trying to add a new entity to my email templates so I have:
  1) I created a new plugin with my custom entity</p>
  
  <pre><code>        $connection-&gt;executeQuery('
              CREATE TABLE IF NOT EXISTS `my_entity` (
                `id` BINARY(16) NOT NULL,
                `message` VARCHAR(255) NOT NULL
              ) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_unicode_ci;
          ');
  </code></pre>
  
  <p>2) I have added the first row in this column:</p>
  
  <pre><code>id      | name
  bin... | test
  </code></pre>
  
  <p>3) I edit this record in db: <code>mail_template_type</code> column <code>available_entities</code>
  <code>{"order":"order","salesChannel":"sales_channel","myEntity":"my_entity"}</code></p>
  
  <p>And now when I edit the email, twig tells me that there is such a variable to use, but when I use it, there is nothing in it <code>{{ myEntity.name }}</code></p>
  
  <p>Can I have a clear explanation, how can I do that?</p>