Allow to add custom fields to the "Manually enabled via checkbox" trigger

[andergmartins]

If the workflow has the "Manually enabled via checkbox" trigger, a checkbox is displayed in the quick-edit and post-edit pages. This only allows us to enable the workflow for the post, but it would be more useful if the user could select custom fields so that we can mimic the legacy Future Action options.

When creating the workflow and configuring that trigger, we could make custom fields by selecting the field type, label, and name. The name would be available as a variable/output provided by the trigger.

This helps to customize the workflow per post.