Add ckeditor iframedialog to set of plugins shipped by default

Info and code for plugin can be found here: https://ckeditor.com/cke4/addon/iframedialog

The Bynder sample ckeditor plugin uses this iframedialog plugin to render the Bynder image picker. Shipping it with the product by default ensures that customers don't have to manually copy and install it.

Documentation also needs to be updated (https://documentation.bloomreach.com/library/concepts/document-types/html-fields/ckeditor-plugins.html).