AI - Support multiple in field widget actions.

Problem/Motivation

The field widget actions plugin currently does not work correctly with multi-value custom fields.

Steps to reproduce

Add a field widget action to an existing multi-valued custom field. While the response can come back with multiple items, the unlimited nature of the field doesn't expose the items to fill. Additionally, the button should appear once for all items vs. per item.

Proposed resolution

• Adapt other AI field plugin implementations to extend the actionButton and add ajax.
• Add multiple: FALSE to the FWA plugin.