feat: improve RCTAppDelegate usage for brownfield (#46625)

Summary:
This PR improves the usage of `RCTAppDelegate` for brownfield scenarios.

Currently, when we want to integrate React Native with a brownfield app users might not want to initialize React Native in the main window. They may want to create it later.

Example usage:

```swift
class AppDelegate: RCTAppDelegate {
   override func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey: Any]?) -> Bool {
      // Disable automatically creating react native window
      self.automaticallyLoadReactNativeWindow = false

      return super.application(application, didFinishLaunchingWithOptions: launchOptions)
  }
}
```

```swift
import Foundation
import React
import React_RCTAppDelegate

class SettingsViewController: UIViewController {

  override func viewDidLoad() {
    super.viewDidLoad()
    self.view = (RCTSharedApplication()?.delegate as? RCTAppDelegate)?.rootViewFactory .view(withModuleName: "Settings", initialProperties: [:])
  }
}
```

## Changelog:

[IOS] [ADDED] - improve RCTAppDelegate usage for brownfield, add `automaticallyLoadReactNativeWindow` flag

Pull Request resolved: https://github.com/facebook/react-native/pull/46625

Test Plan: CI Green

Reviewed By: cortinico

Differential Revision: D63325397

Pulled By: cipolleschi

fbshipit-source-id: 1361bda5fcd91f4933219871c64a84a83c281c34