The kind of widget to use with the generic plugin is specified with the widgetType property in the client config for the plugin. Any configuration for the widget is given in the config property, and may vary depending on the particular widget. If you don't specify the widgetType to use, the String widget is used by default.
The plugin currently supports these widgets:
The data contract between the server and the widget is a JSON object. By default, the JSON object will be passed as-is to the widget for display. You can use the fieldName property to select some property within the JSON structure. It is a whitespace separated string denoting how to traverse the JSON object to arrive at the desired property. You can also use the transform property to specify a function to apply to the JSON object, to map it to another JSON object. The function is applied before fieldName selection, so you could in principle do both.