Add exec Node API to FlipperLib

Summary: Changelog: Allow flipper plugins to run "exec" Node API on Flipper server.

Reviewed By: mweststrate

Differential Revision: D32881149

fbshipit-source-id: 46486a47ee9824ca68897c19fd86b4afc7f8bf1d

desktop/flipper-server/src/startSocketServer.tsx

@@ -36,6 +36,7 @@ export function startSocketServer(
         })
         .catch((error: any) => {
           if (connected) {
+            // TODO: Serialize error
             client.emit('exec-response-error', id, error.toString());
           }
         });