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

@@ -400,6 +400,7 @@ export function createMockFlipperLib(options?: StartPluginOptions): FlipperLib {
         mkdir: jest.fn(),
         copyFile: jest.fn(),
       },
+      downloadFile: jest.fn(),
     },
   };
 }