feat: allow custom assignment of rootView to rootViewController (#37873)

Summary:
To use a native Drawer on iPad, I can override `createRootViewController` to create a `UISplitViewController` instead of a `UIViewController`, but I then need to assign the rootView with
```objective-c
[splitViewController setViewController:mainVC forColumn:UISplitViewControllerColumnSecondary];
```
which I can currently only do by copy pasting the entire `didFinishLaunchingWithOptions` and only replacing the assignment
```objective-c
    rootViewController.view = rootView;
```

In an attempt of making it easier for developers to use a native drawer in iOS, being able to override the assignment would make it easier.

bypass-github-export-checks

## Changelog:
[iOS] [ADDED] - added override method with default implementation

Pull Request resolved: https://github.com/facebook/react-native/pull/37873

Test Plan: Tested on iPad iOS 16 simulator

Reviewed By: cortinico

Differential Revision: D46761919

Pulled By: cipolleschi

fbshipit-source-id: c3ece0170d732133edc08f220a2f9a67da93815a

packages/react-native/Libraries/AppDelegate/RCTAppDelegate.h

@@ -36,6 +36,7 @@
  *   - (UIView *)createRootViewWithBridge:(RCTBridge *)bridge moduleName:(NSString*)moduleName initProps:(NSDictionary
  *)initProps;
  *   - (UIViewController *)createRootViewController;
+ *   - (void)setRootView:(UIView *)rootView toRootViewController:(UIViewController *)rootViewController;
  * New Architecture:
  *   - (BOOL)concurrentRootEnabled
  *   - (BOOL)turboModuleEnabled;
@@ -94,6 +95,16 @@
  */
 - (UIViewController *)createRootViewController;
 
+/**
+ * It assigns the rootView to the rootViewController
+ * By default, it assigns the rootView to the view property of the rootViewController
+ * If you are not using a simple UIViewController, then there could be other methods to use to setup the rootView.
+ * For example: UISplitViewController requires `setViewController(_:for:)`
+ *
+ * @return: void
+ */
+- (void)setRootView:(UIView *)rootView toRootViewController:(UIViewController *)rootViewController;
+
 /// This method controls whether the App will use RuntimeScheduler. Only applicable in the legacy architecture.
 ///
 /// @return: `YES` to use RuntimeScheduler, `NO` to use JavaScript scheduler. The default value is `YES`.

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

@@ -77,7 +77,7 @@ static NSString *const kRNConcurrentRoot = @"concurrentRoot";
 
   self.window = [[UIWindow alloc] initWithFrame:[UIScreen mainScreen].bounds];
   UIViewController *rootViewController = [self createRootViewController];
-  rootViewController.view = rootView;
+  [self setRootView:rootView toRootViewController:rootViewController];
   self.window.rootViewController = rootViewController;
   [self.window makeKeyAndVisible];
 
@@ -129,6 +129,11 @@ static NSString *const kRNConcurrentRoot = @"concurrentRoot";
   return [UIViewController new];
 }
 
+- (void)setRootView:(UIView *)rootView toRootViewController:(UIViewController *)rootViewController
+{
+  rootViewController.view = rootView;
+}
+
 - (BOOL)runtimeSchedulerEnabled
 {
   return YES;