feat: add initialProps property to RCTAppDelegate (#35848)

Summary:
Hi there,

While upgrading to 0.71 we realised the RCTAppDelegate doesn't offer a way to set custom `initProps` that would depend on `launchOptions`.

This PR adds an `initialProps` property to the RCTAppDelegate. This would let us set this property based on `launchOptions` in our implementation of `didFinishLaunchingWithOptions` before calling `[super didFinishLaunchingWithOptions]`

Thanks !

## Changelog

[IOS] [ADDED] - Add `initialProps` property to `RCTAppDelegate`

Pull Request resolved: https://github.com/facebook/react-native/pull/35848

Reviewed By: rshest

Differential Revision: D42543027

Pulled By: cipolleschi

fbshipit-source-id: 55374914890445f8193c12a06a943b7796edb457

Libraries/AppDelegate/RCTAppDelegate.h

@@ -55,6 +55,7 @@
 @property (nonatomic, strong) UIWindow *window;
 @property (nonatomic, strong) RCTBridge *bridge;
 @property (nonatomic, strong) NSString *moduleName;
+@property (nonatomic, strong) NSDictionary *initialProps;
 
 /**
  * It creates a `RCTBridge` using a delegate and some launch options.

Libraries/AppDelegate/RCTAppDelegate.mm

@@ -84,7 +84,7 @@ static NSString *const kRNConcurrentRoot = @"concurrentRoot";
 
 - (NSDictionary *)prepareInitialProps
 {
-  NSMutableDictionary *initProps = [NSMutableDictionary new];
+  NSMutableDictionary *initProps = self.initialProps ? [self.initialProps mutableCopy] : [NSMutableDictionary new];
 
 #ifdef RCT_NEW_ARCH_ENABLED
   initProps[kRNConcurrentRoot] = @([self concurrentRootEnabled]);

template/ios/HelloWorld/AppDelegate.mm

@@ -7,6 +7,10 @@
 - (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions
 {
   self.moduleName = @"HelloWorld";
+  // You can add your custom initial props in the dictionary below.
+  // They will be passed down to the ViewController used by React Native.
+  self.initialProps = @{};
+
   return [super application:application didFinishLaunchingWithOptions:launchOptions];
 }