feat(custom-widgets): complete Custom JSX v2 (#6390)

packages/api/src/custom-widget-configuration.ts

@@ -0,0 +1,6 @@
+export * from "./router/custom-widget/configuration-requests";
+export { configurePreviewSessionSource } from "./router/custom-widget/preview-sessions";
+export {
+  parseStoredCustomWidgetDefinition,
+  serializeCustomWidgetDefinition,
+} from "./router/custom-widget/stored-definition";