Add remote fs API

Summary: Changelog: Expose Flipper Server FS access to Flipper plugins

Reviewed By: lblasa

Differential Revision: D32883144

fbshipit-source-id: 47637b61849ef60a2d8fe91a0a28d2a358e0b8c4

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

@@ -393,7 +393,13 @@ export function createMockFlipperLib(options?: StartPluginOptions): FlipperLib {
       childProcess: {
         exec: jest.fn(),
       },
-      fs: {},
+      fs: {
+        access: jest.fn(),
+        pathExists: jest.fn(),
+        unlink: jest.fn(),
+        mkdir: jest.fn(),
+        copyFile: jest.fn(),
+      },
     },
   };
 }