Introduce showNotification API

Summary: Introduced `showNotifcation` to the Sandy API.

Reviewed By: jknoxville

Differential Revision: D27012001

fbshipit-source-id: d3f237910a478400b0f925f0362af485c96072bb

desktop/flipper-plugin/src/test-utils/test-utils.tsx

@@ -371,6 +371,7 @@ export function createMockFlipperLib(options?: StartPluginOptions): FlipperLib {
     selectPlugin: jest.fn(),
     isPluginAvailable: jest.fn().mockImplementation(() => false),
     writeTextToClipboard: jest.fn(),
+    showNotification: jest.fn(),
   };
 }