[FIXME(links)] ios: Add workaround for package:firebase_messaging when building with Xcode 16

Without this workaround iOS build currently fails with
the following error:

  Lexical or Preprocessor Issue (Xcode): Include of non-modular header inside framework module 'firebase_messaging FLTFirebaseMessagingPlugin':
  '/Users/rajveer/Projects/zulip-flutter/ios/Pods/Headers/Public/Firebase/Firebase.h'
  /Users/rajveer/.pub-cache/hosted/pub.dev/firebase_messaging-15.0.4/ios/Classes/FLTFirebaseMessagingPlugin.h:11:8

Upstream issue and the workaround comment:
  https://github.com/firebase/flutterfire/issues/ 13323
  https://github.com/firebase/flutterfire/issues/ 13323#issuecomment-2355013360

Docs:
  https://developer.apple.com/documentation/xcode/build-settings-reference#Allow-Non-modular-Includes-In-Framework-Modules

The documentation states that enabling this setting may
result in issues later on, which is why this workaround
should only be temporary until firebase_messaging is
patched with the correct fix.

Author: rajveermalviya

ios/Flutter/Debug.xcconfig

@@ -1,2 +1,3 @@
 #include? "Pods/Target Support Files/Pods-Runner/Pods-Runner.debug.xcconfig"
 #include "Generated.xcconfig"
+#include "Zulip.xcconfig"

ios/Flutter/Release.xcconfig

@@ -1,2 +1,3 @@
 #include? "Pods/Target Support Files/Pods-Runner/Pods-Runner.release.xcconfig"
 #include "Generated.xcconfig"
+#include "Zulip.xcconfig"

ios/Flutter/Zulip.xcconfig

@@ -0,0 +1,7 @@
+// TODO(firebase/flutterfire#13323): remove this flag
+//
+// This flag is added to workaround the iOS build failing
+// on Xcode 16, remove it when `package:firebase_messaging`
+// is patched with a fix to build successfully on Xcode 16.
+//
+CLANG_ALLOW_NON_MODULAR_INCLUDES_IN_FRAMEWORK_MODULES=YES