Add download file API

Summary: Changelog: Expose "downloadFile" API to Flipper plugins. Allow them to download files form the web to Flipper Server.

Reviewed By: mweststrate

Differential Revision: D32950685

fbshipit-source-id: 7b7f666e165ff7bf209230cdc96078272ede3616

desktop/flipper-common/src/index.tsx

@@ -20,6 +20,7 @@ export * from './server-types';
 export {sleep} from './utils/sleep';
 export {timeout} from './utils/timeout';
 export {isTest} from './utils/isTest';
+export {assertNever} from './utils/assertNever';
 export {
   logPlatformSuccessRate,
   reportPlatformFailures,