Issue #3380326 by kriceiq: adds submit_button_value to rest output

src/Controller/SimpleSurveyRestController.php

@@ -286,6 +286,7 @@ class SimpleSurveyRestController extends ControllerBase {
         'inputType' => $question->get('input_type')->value,
         'displayAs' => $question->get('display_as')->value,
         'questionType' => $question->get('question_type')->value,
+        'submit_button_value' => $question->get('submit_button_value')->value,
         'id' => $genericId,
         'disclaimerText' => [
           'en' => $question->get('question_disclaimer')->value,