Create Test Ajax Submission Handler

Problem

As WebformHandler should support Ajax callbacks. While working on #2854021: Send email based on element options selection, I ran into major issue with #ajax within handler's configuration form.

Solution

Build a 'test_ajax' handler. (\Drupal\webform_test_handler\Plugin\WebformHandler\TestAjaxWebformHandler)

Tasks

• Setup handler
• Test entity type to fields mapping.
• Test add and remove.
• Setup basic mapping example.

References

• https://www.mike-miles.com/blog/creating-ajax-callback-commands-drupal-8
• https://www.sitepoint.com/using-ajax-forms-drupal-8/