Add flag to enable building Hermes from source on iOS

Summary:
If Hermes is enabled on a project and the envvar `BUILD_HERMES_SOURCE` is truthy, a React Native project will build Hermes from source when running `pod install`:

```
cd ios/
BUILD_HERMES_SOURCE=1 pod install
```

* Requires `cmake` and `ninja` to be installed: `brew install cmake ninja`.
* Requires a `react-native` release with Hermes source code bundled in (`scripts/publish-npm.js --include-hermes && npm pack`).

# Changelog

[Internal]

Reviewed By: cortinico

Differential Revision: D34313581

fbshipit-source-id: 4ba5f82aa6c7027fd37f1982127fc5f63d931ded

Author: hramos

scripts/react_native_pods.rb

@@ -109,7 +109,13 @@ def use_react_native! (options={})
 
   if hermes_enabled
     pod 'React-hermes', :path => "#{prefix}/ReactCommon/hermes"
-    pod 'hermes-engine', '~> 0.11.0'
+
+    if ENV['BUILD_HERMES_SOURCE'] == '1'
+      hermes_source_path = locatePathToHermesSource!(prefix)
+      pod 'hermes-engine', :path => "#{hermes_source_path}/hermes-engine.podspec"
+    else
+      pod 'hermes-engine', '~> 0.11.0'
+    end
     pod 'libevent', '~> 2.1.12'
   end
 end
@@ -622,6 +628,20 @@ def use_react_native_codegen!(spec, options={})
   }
 end
 
+def locatePathToHermesSource!(react_native_path)
+  return if ENV['BUILD_HERMES_SOURCE'] != '1'
+
+  hermes_source_path = "#{react_native_path}/sdks/hermes"
+
+  if !File.exist?(hermes_source_path)
+    Pod::UI.warn "[Hermes] Error: Hermes source code must be present at #{hermes_source_path} when BUILD_HERMES_SOURCE is enabled."
+    Pod::UI.warn '[Hermes] Verify that the release of React Native that you are using has the Hermes source code included.'
+    exit 1
+  end
+
+  hermes_source_path
+end
+
 # This provides a post_install workaround for build issues related Xcode 12.5 and Apple Silicon (M1) machines.
 # Call this in the app's main Podfile's post_install hook.
 # See https://github.com/facebook/react-native/issues/31480#issuecomment-902912841 for more context.