Add view getter on RCTRootView / RCTFabricSurfaceHostingProxyRootView (#37310)

Summary:
Hi 👋

During the [react-native-bootsplash](https://github.com/zoontek/react-native-bootsplash) implementation of the new architecture, I noticed a few thing regarding `RCTRootView` / `RCTFabricSurfaceHostingProxyRootView` compat.

Currently `RCTRootView` inherits from `UIView`, but `RCTFabricSurfaceHostingProxyRootView` does not, which this works:

```obj-c
- (UIView *)createRootViewWithBridge:(RCTBridge *)bridge
                          moduleName:(NSString *)moduleName
                           initProps:(NSDictionary *)initProps {
  RCTRootView *rootView = (RCTRootView *)
      [super createRootViewWithBridge:bridge moduleName:moduleName initProps:initProps];

  UIStoryboard *storyboard = [UIStoryboard storyboardWithName:@"LaunchScreen" bundle:nil];
  UIView *loadingView = [[storyboard instantiateInitialViewController] view];

  loadingView.autoresizingMask = UIViewAutoresizingFlexibleWidth | UIViewAutoresizingFlexibleHeight;
  loadingView.frame = rootView.bounds;
  loadingView.center = (CGPoint){CGRectGetMidX(rootView.bounds), CGRectGetMidY(rootView.bounds)};
  loadingView.hidden = NO;

  [rootView addSubview:loadingView];

  return rootView;
}
```

But this doesn't:

```obj-c
- (UIView *)createRootViewWithBridge:(RCTBridge *)bridge
                          moduleName:(NSString *)moduleName
                           initProps:(NSDictionary *)initProps {
  RCTFabricSurfaceHostingProxyRootView *rootView = (RCTFabricSurfaceHostingProxyRootView *)
      [super createRootViewWithBridge:bridge moduleName:moduleName initProps:initProps];

  UIStoryboard *storyboard = [UIStoryboard storyboardWithName:@"LaunchScreen" bundle:nil];
  UIView *loadingView = [[storyboard instantiateInitialViewController] view];

  loadingView.autoresizingMask = UIViewAutoresizingFlexibleWidth | UIViewAutoresizingFlexibleHeight;
  loadingView.frame = rootView.bounds;
  loadingView.center = (CGPoint){CGRectGetMidX(rootView.bounds), CGRectGetMidY(rootView.bounds)};
  loadingView.hidden = NO;

  [rootView addSubview:loadingView];

  return rootView;
}
```

Because `RCTFabricSurfaceHostingProxyRootView` is an imperfect proxy as it doesn't give access to the underlaying `UIView *`. As a solution, I added a prop on both: `UIView *view`

PS: I'm well aware that `setLoadingView` also exists in both files, but it's currently not usable as the current `isActivityIndicatorViewVisible` / `isSurfaceViewVisible` / `_activityIndicatorViewFactory` logic in `RCTSurfaceHostingView.mm` doesn't work: a situation where `isActivityIndicatorViewVisible == true && isSurfaceViewVisible == false && _activityIndicatorViewFactory != nil` never happen:

<img width="1162" alt="Screenshot_2023-05-06_at_18 10 18" src="https://user-images.githubusercontent.com/1902323/236883439-2256ddfb-7846-482a-b957-002a7d51a148.png">

## Changelog:

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Pick one each for the category and type tags:

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

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Pull Request resolved: https://github.com/facebook/react-native/pull/37310

Test Plan:
Add this block of code in `AppDelegate.mm`:

```obj-c
#import <React/RCTRootView.h>

#if __has_include(<React/RCTFabricSurfaceHostingProxyRootView.h>)
#import <React/RCTFabricSurfaceHostingProxyRootView.h>
#endif

// …

- (UIView *)createRootViewWithBridge:(RCTBridge *)bridge
                          moduleName:(NSString *)moduleName
                           initProps:(NSDictionary *)initProps {
#ifdef RCT_NEW_ARCH_ENABLED
  RCTFabricSurfaceHostingProxyRootView *rootView = (RCTFabricSurfaceHostingProxyRootView *)
#else
  RCTRootView *rootView = (RCTRootView *)
#endif
      [super createRootViewWithBridge:bridge moduleName:moduleName initProps:initProps];

  // accessing the "real" root view on both arch
  UIView *view = rootView.view;

  UIStoryboard *storyboard = [UIStoryboard storyboardWithName:@"LaunchScreen" bundle:nil];
  UIView *loadingView = [[storyboard instantiateInitialViewController] view];

  loadingView.autoresizingMask = UIViewAutoresizingFlexibleWidth | UIViewAutoresizingFlexibleHeight;
  loadingView.frame = view.bounds;
  loadingView.center = (CGPoint){CGRectGetMidX(view.bounds), CGRectGetMidY(view.bounds)};
  loadingView.hidden = NO;

  [view addSubview:loadingView];

  return rootView;
}
```

It should persist the splash screen on both old and new architecture.

Reviewed By: sammy-SC

Differential Revision: D45688644

Pulled By: cipolleschi

fbshipit-source-id: b6f2fc8091a15189ea2eceb8ea426593f62674cb

packages/react-native/React/Base/RCTRootView.h

@@ -133,6 +133,11 @@ extern
  */
 @property (nonatomic, weak, nullable) UIViewController *reactViewController;
 
+/**
+ * The root view casted as UIView. Used by splash screen libraries.
+ */
+@property (nonatomic, strong, readonly) UIView *view;
+
 /**
  * The React-managed contents view of the root view.
  */

packages/react-native/React/Base/RCTRootView.m

@@ -116,6 +116,11 @@ NSString *const RCTContentDidAppearNotification = @"RCTContentDidAppearNotificat
 RCT_NOT_IMPLEMENTED(-(instancetype)initWithFrame : (CGRect)frame)
 RCT_NOT_IMPLEMENTED(-(instancetype)initWithCoder : (NSCoder *)aDecoder)
 
+- (UIView *)view
+{
+  return self;
+}
+
 - (BOOL)hasBridge
 {
   return _bridge != nil;

packages/react-native/React/Base/Surface/SurfaceHostingView/RCTSurfaceHostingProxyRootView.h

@@ -34,6 +34,7 @@ NS_ASSUME_NONNULL_BEGIN
 @property (nonatomic, assign) RCTRootViewSizeFlexibility sizeFlexibility;
 @property (nonatomic, weak) id<RCTRootViewDelegate> delegate;
 @property (nonatomic, weak) UIViewController *reactViewController;
+@property (nonatomic, strong, readonly) UIView *view;
 @property (nonatomic, strong, readonly) UIView *contentView;
 @property (nonatomic, strong) UIView *loadingView;
 @property (nonatomic, assign) BOOL passThroughTouches;

packages/react-native/React/Base/Surface/SurfaceHostingView/RCTSurfaceHostingProxyRootView.mm

@@ -134,6 +134,11 @@ RCT_NOT_IMPLEMENTED(-(instancetype)initWithCoder : (NSCoder *)aDecoder)
   return super.surface.moduleName;
 }
 
+- (UIView *)view
+{
+  return (UIView *)super.surface.view;
+}
+
 - (UIView *)contentView
 {
   return self;