feat(iOS): allow eager initialization of RCTRootViewFactory (#49986)

Summary:
Expose eager initialization method on `RCTRootViewFactory` (iOS) so that application can prepare `ReactHost`/Bridge before actually creating a root view. Then creating a root view is significantly faster.

## Changelog:
[IOS] [ADDED] - allow eager initialization of `RCTRootViewFactory`

<!-- Help reviewers and the release process by writing your own changelog entry.

Pick one each for the category and type tags:

[ANDROID|GENERAL|IOS|INTERNAL] [BREAKING|ADDED|CHANGED|DEPRECATED|REMOVED|FIXED|SECURITY] - Message

For more details, see:
https://reactnative.dev/contributing/changelogs-in-pull-requests

Pull Request resolved: https://github.com/facebook/react-native/pull/49986

Test Plan:
Invoke `initializeReactHostWithLaunchOptions:` before calling `viewWithModuleName:` and measure the time difference vs not using eager initilization:

Before
- calling `viewWithModuleName:`: 63.39ms, 47.91 ms, 60.18ms

After:
- calling `initializeReactHostWithLaunchOptions`: 52.41 ms, 81.03 ms, 60.52 ms
- calling `viewWithModuleName`: 0.49 ms,  0.63 ms, 0.47 ms

Test run 3 times on iPhone simulator on M1 mac.

Reviewed By: javache

Differential Revision: D71548601

Pulled By: cipolleschi

fbshipit-source-id: 86ecfb8bec4c2657537caf32af49545b21d3656b

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

@@ -212,6 +212,15 @@ typedef void (^RCTLoadSourceForBridgeBlock)(RCTBridge *bridge, RCTSourceLoadBloc
 
 #pragma mark - RCTRootViewFactory Helpers
 
+/**
+ * Initialize React Host/Bridge without creating a view.
+ *
+ * Use it to speed up later viewWithModuleName: calls.
+ *
+ * @parameter: launchOptions  - a dictionary with a set of options.
+ */
+- (void)initializeReactHostWithLaunchOptions:(NSDictionary *__nullable)launchOptions;
+
 - (RCTHost *)createReactHost:(NSDictionary *__nullable)launchOptions;
 
 @end

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

@@ -132,9 +132,7 @@
   return [self viewWithModuleName:moduleName initialProperties:nil launchOptions:nil];
 }
 
-- (UIView *)viewWithModuleName:(NSString *)moduleName
-             initialProperties:(NSDictionary *)initProps
-                 launchOptions:(NSDictionary *)launchOptions
+- (void)initializeReactHostWithLaunchOptions:(NSDictionary *)launchOptions
 {
   if (_configuration.bridgelessEnabled) {
     // Enable TurboModule interop by default in Bridgeless mode
@@ -142,7 +140,20 @@
     RCTEnableTurboModuleInteropBridgeProxy(YES);
 
     [self createReactHostIfNeeded:launchOptions];
+    return;
+  }
 
+  [self createBridgeIfNeeded:launchOptions];
+  [self createBridgeAdapterIfNeeded];
+}
+
+- (UIView *)viewWithModuleName:(NSString *)moduleName
+             initialProperties:(NSDictionary *)initProps
+                 launchOptions:(NSDictionary *)launchOptions
+{
+  [self initializeReactHostWithLaunchOptions:launchOptions];
+
+  if (_configuration.bridgelessEnabled) {
     RCTFabricSurface *surface = [self.reactHost createSurfaceWithModuleName:moduleName initialProperties:initProps];
 
     RCTSurfaceHostingProxyRootView *surfaceHostingProxyRootView =
@@ -155,9 +166,6 @@
     return surfaceHostingProxyRootView;
   }
 
-  [self createBridgeIfNeeded:launchOptions];
-  [self createBridgeAdapterIfNeeded];
-
   UIView *rootView;
   if (_configuration.createRootViewWithBridge != nil) {
     rootView = _configuration.createRootViewWithBridge(self.bridge, moduleName, initProps);