Issue #3213195: Support configuration for checkboxes/radio buttons widget

config/schema/pluginreference.schema.yml

@@ -127,3 +127,11 @@ field.widget.settings.plugin_reference_select:
     configuration_form:
       type: string
       label: 'How the plugin configuration form will be shown.'
+
+field.widget.settings.plugin_reference_options_buttons:
+  type: mapping
+  label: 'Default pluginreference widget settings'
+  mapping:
+    configuration_form:
+      type: string
+      label: 'How the plugin configuration form will be shown.'