Implement subscribing to data updates for Flipper Server Companion

Summary: Allow subscribing to state updates from the plugin in headless mode

Reviewed By: passy

Differential Revision: D36516754

fbshipit-source-id: 14db51243e1d91332a7327c1792412149339f907

desktop/flipper-common/src/index.tsx

@@ -17,6 +17,7 @@ export {
   NoopLogger,
 } from './utils/Logger';
 export * from './server-types';
+export * from './companion-types';
 export * from './ServerAddOn';
 export {sleep} from './utils/sleep';
 export {timeout} from './utils/timeout';