Introduce menu entry support

Summary:
[interesting] since it shows how Flipper APIs are exposed through sandy. However, the next diff is a much simpler example of that

This diff adds support for adding menu entries for sandy plugin (renamed keyboard actions to menus, as it always creates a menu entry, but not necessarily a keyboard shortcut)

```

  client.addMenuEntry(
    // custom entry
    {
      label: 'Reset Selection',
      topLevelMenu: 'Edit',
      handler: () => {
        selectedID.set(null);
      },
    },
    // based on built-in action (sets standard label, shortcut)
    {
      action: 'createPaste',
      handler: () => {
        console.log('creating paste');
      },
    },
  );
```

Most of this diff is introducing the concept of FlipperUtils, a set of static Flipper methods (not related to a device or client) that can be used from Sandy. This will for example be used to implement things as `createPaste` as well

Reviewed By: nikoant

Differential Revision: D22766990

fbshipit-source-id: ce90af3b700e6c3d9a779a3bab4673ba356f3933

desktop/app/src/test-utils/createMockFlipperWithPlugin.tsx

@@ -16,6 +16,7 @@ import {
   act as testingLibAct,
 } from '@testing-library/react';
 import {queries} from '@testing-library/dom';
+import {TestUtils} from 'flipper-plugin';
 
 import {
   selectPlugin,
@@ -36,6 +37,7 @@ import {registerPlugins} from '../reducers/plugins';
 import PluginContainer from '../PluginContainer';
 import {getPluginKey, isDevicePluginDefinition} from '../utils/pluginUtils';
 import {getInstance} from '../fb-stubs/Logger';
+import {setFlipperLibImplementation} from '../utils/flipperLibImplementation';
 
 type MockFlipperResult = {
   client: Client;
@@ -62,6 +64,8 @@ export async function createMockFlipperWithPlugin(
 ): Promise<MockFlipperResult> {
   const store = createStore(rootReducer);
   const logger = getInstance();
+  setFlipperLibImplementation(TestUtils.createMockFlipperLib());
+
   store.dispatch(registerPlugins([pluginClazz]));
 
   function createDevice(serial: string): BaseDevice {