Expose unstable_InspectorProxy and unstable_Device from dev-middleware (#41370)

Summary:
Recently, both `metro-inspector-proxy`(https://github.com/facebook/react-native/pull/39045) and `react-native-community/cli-plugin-metro`(https://github.com/facebook/react-native/pull/38795) were moved to this repo and in the process of moving these packages, the `exports` field inside package.json was added, only exporting the `index.js` file.

The problem is that Expo CLI (and possibly other community packages) rely on functions and classes that are not exported in the `index.js` file, e.g. Importing the InspectorProxy class from `react-native/dev-middleware/dist/inspector-proxy/InspectorProxy`. Normally this wouldn't be a problem and we would just import from `dist/` but due to the `exports` field, attempting to import from any other file not specified on this field will result in a `ERR_PACKAGE_PATH_NOT_EXPORTED` error.

As a short-term fix, we should create `unstable_`-prefixed exports of individual features Expo currently depends on.

## Changelog:

[INTERNAL] [CHANGED] - Expose unstable_InspectorProxy and unstable_Device from `react-native/dev-middleware`

Pull Request resolved: https://github.com/facebook/react-native/pull/41370

Test Plan: N / A

Reviewed By: robhogan

Differential Revision: D51163134

Pulled By: blakef

fbshipit-source-id: e67adaedc4fc64131e4c9dd8383c9877b8202283

packages/dev-middleware/src/createDevMiddleware.js

@@ -60,6 +60,13 @@ type Options = $ReadOnly<{
    * This is an unstable API with no semver guarantees.
    */
   unstable_experiments?: ExperimentsConfig,
+
+  /**
+   * An interface for using a modified inspector proxy implementation.
+   *
+   * This is an unstable API with no semver guarantees.
+   */
+  unstable_InspectorProxy?: Class<InspectorProxy>,
 }>;
 
 type DevMiddlewareAPI = $ReadOnly<{
@@ -74,10 +81,12 @@ export default function createDevMiddleware({
   unstable_browserLauncher = DefaultBrowserLauncher,
   unstable_eventReporter,
   unstable_experiments: experimentConfig = {},
+  unstable_InspectorProxy,
 }: Options): DevMiddlewareAPI {
   const experiments = getExperiments(experimentConfig);
 
-  const inspectorProxy = new InspectorProxy(
+  const InspectorProxyClass = unstable_InspectorProxy ?? InspectorProxy;
+  const inspectorProxy = new InspectorProxyClass(
     projectRoot,
     serverBaseUrl,
     unstable_eventReporter,

packages/dev-middleware/src/index.flow.js

@@ -13,3 +13,6 @@ export {default as createDevMiddleware} from './createDevMiddleware';
 
 export type {BrowserLauncher, LaunchedBrowser} from './types/BrowserLauncher';
 export type {EventReporter, ReportableEvent} from './types/EventReporter';
+
+export {default as unstable_InspectorProxy} from './inspector-proxy/InspectorProxy';
+export {default as unstable_Device} from './inspector-proxy/Device';