Automate setting the RCTNewArchEnabled flag (#49927)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/49927

This change automates updating App's Info.plist with the new `RCTNewArchEnabled` boolean entry. The value depends on how the pod install is set up. In this way, we maintain the previous UX to enable/disable the New Arch.

## Context
The RCT_NEW_ARCH_ENABLE flag is a compile time flag we used for almost two years to configure the iOS apps and to determine whether the app should build with the New Arch or not.

However, given that we are looking into prebuilding React Native, we have to get rid of all the compilation flags, because they would require us to prebuild a combinatorial number of artifacts for react native. For example:

- New Arch / Hermes
- Old Arch / Hermes
- New Arch / JSC
- Old Arch / JSC
- ...

## Backward compatibility
We are going to keep adding the RCT_NEW_ARCH_ENABLED flag in all the dependencies, through the cocoapods inrastructure, so libraries, which are not prebuilt, will be build for the right architecture by the app itself.

## Changelog:
[iOS][Added] -

Reviewed By: cortinico

Differential Revision: D70888212

fbshipit-source-id: 541a818e02ccb96c822de29b0714646d1e69a5a9

packages/react-native/scripts/cocoapods/new_architecture.rb

@@ -9,6 +9,7 @@ require_relative "./utils.rb"
 require_relative "./helpers.rb"
 require_relative "./jsengine.rb"
 
+
 class NewArchitectureHelper
     @@NewArchWarningEmitted = false # Used not to spam warnings to the user.
 
@@ -159,4 +160,40 @@ class NewArchitectureHelper
     def self.new_arch_enabled
         return ENV["RCT_NEW_ARCH_ENABLED"] == '0' ? false : true
     end
+
+    def self.set_RCTNewArchEnabled_in_info_plist(installer, new_arch_enabled)
+        projectPaths = installer.aggregate_targets
+            .map{ |t| t.user_project }
+            .uniq{ |p| p.path }
+            .map{ |p| p.path }
+
+        excluded_info_plist = ["/Pods", "Tests", "metainternal", ".bundle"]
+        projectPaths.each do |projectPath|
+            projectFolderPath = File.dirname(projectPath)
+            infoPlistFiles = `find #{projectFolderPath} -name "Info.plist"`
+            infoPlistFiles = infoPlistFiles.split("\n").map { |f| f.strip }
+
+            infoPlistFiles.each do |infoPlistFile|
+                # If infoPlistFile contains Pods or tests, skip it
+                should_skip = false
+                excluded_info_plist.each do |excluded|
+                    if infoPlistFile.include? excluded
+                        should_skip = true
+                    end
+                end
+                next if should_skip
+
+                # Read the file as a plist
+                info_plist = Xcodeproj::Plist.read_from_path(infoPlistFile)
+                # Check if it contains the RCTNewArchEnabled key
+                if info_plist["RCTNewArchEnabled"] and info_plist["RCTNewArchEnabled"] == new_arch_enabled
+                    next
+                end
+
+                # Add the key and value to the plist
+                info_plist["RCTNewArchEnabled"] = new_arch_enabled ? true : false
+                Xcodeproj::Plist.write_to_path(info_plist, infoPlistFile)
+            end
+        end
+    end
 end

packages/react-native/scripts/react_native_pods.rb

@@ -451,6 +451,7 @@ def react_native_post_install(
 
   NewArchitectureHelper.set_clang_cxx_language_standard_if_needed(installer)
   NewArchitectureHelper.modify_flags_for_new_architecture(installer, NewArchitectureHelper.new_arch_enabled)
+  NewArchitectureHelper.set_RCTNewArchEnabled_in_info_plist(installer, NewArchitectureHelper.new_arch_enabled)
 
   if ENV['USE_HERMES'] == '0' && ENV['USE_THIRD_PARTY_JSC'] != '1'
     print_jsc_removal_message()