Add missing loadFromSource method in the DefaultRNFactoryDelegate (#49931)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/49931

This change fixes the app startup in the Old Architecture by implementing the loadSourceForBridge:onProgress:onComplete method in the RCTDefaultReactNativeFactoryDelegate object.

The method was missing here, so the Bridge was never trying to load the JS bundle from Metro, resulting in an empty app.

## Changelog:
[iOS][Fixed] - Implement the loadSourceForBridge:onProgress:onComplete in the RCTDefaultReactNativeFactoryDelegate.

Reviewed By: cortinico

Differential Revision: D70898811

fbshipit-source-id: 3e5d519a1965e92ace91ca6d5b316a9069279448

packages/react-native/Libraries/AppDelegate/RCTDefaultReactNativeFactoryDelegate.mm

@@ -118,4 +118,11 @@
   return nullptr;
 }
 
+- (void)loadSourceForBridge:(RCTBridge *)bridge
+                 onProgress:(RCTSourceLoadProgressBlock)onProgress
+                 onComplete:(RCTSourceLoadBlock)loadCallback
+{
+  [RCTJavaScriptLoader loadBundleAtURL:[self sourceURLForBridge:bridge] onProgress:onProgress onComplete:loadCallback];
+}
+
 @end

packages/rn-tester/RNTester/AppDelegate.mm

@@ -76,13 +76,6 @@ static NSString *kBundlePath = @"js/RNTesterApp.ios";
   return [RCTLinkingManager application:app openURL:url options:options];
 }
 
-- (void)loadSourceForBridge:(RCTBridge *)bridge
-                 onProgress:(RCTSourceLoadProgressBlock)onProgress
-                 onComplete:(RCTSourceLoadBlock)loadCallback
-{
-  [RCTJavaScriptLoader loadBundleAtURL:[self sourceURLForBridge:bridge] onProgress:onProgress onComplete:loadCallback];
-}
-
 - (std::shared_ptr<facebook::react::TurboModule>)getTurboModule:(const std::string &)name
                                                       jsInvoker:(std::shared_ptr<facebook::react::CallInvoker>)jsInvoker
 {