title: Typeorm specify data length of simple-json column in MySql
body: <p>I'm using typeorm in my application and I have a simple-json column in my models that can get quite large in some scenarios. In the MySql DB I want this set to <code>LONGTEXT</code> however once doing so all migrations try to revert this back to <code>TEXT</code> and try dropping the column. Is there a way to use the simple-json column type as a longtext in my MySql DB? </p>
  
  <p>Example Migration generation</p>
  
  <p>up:</p>
  
  <pre><code>await queryRunner.query("ALTER TABLE `results` DROP COLUMN `rawData`", undefined);
  await queryRunner.query("ALTER TABLE `results` ADD `rawData` text NULL", undefined);
  </code></pre>
  
  <p>down:</p>
  
  <pre><code>await queryRunner.query("ALTER TABLE `results` DROP COLUMN `rawData`", undefined);
  await queryRunner.query("ALTER TABLE `results` ADD `rawData` longtext NULL", undefined);
  </code></pre>