Expose the submit label of the WebformActions element.

Closes #3419875

graphql/webform.extension.graphqls

@@ -2,6 +2,10 @@ extend type Query {
   webformById(id: String!): Webform
 }
 
+extend type WebformElementWebformActions {
+  submitLabel: String
+}
+
 extend type WebformElementTextarea {
   rows: Int
 }