Implement sending messages from desktop to add-on

Reviewed By: antonk52

Differential Revision: D34239803

fbshipit-source-id: 1a18d522ba877ade3d0703fbb374c15b596241d7

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

@@ -647,5 +647,6 @@ function createServerAddOnControlsMock(): ServerAddOnControls {
   return {
     start: createStubFunction(),
     stop: createStubFunction(),
+    sendMessage: createStubFunction(),
   };
 }