[iOS] Add extensibility point on react-native-xcode.sh script

Original pull request: #12863

Author: brunolemos

scripts/react-native-xcode.sh

@@ -120,7 +120,8 @@ $NODE_BINARY $CLI_PATH $BUNDLE_COMMAND \
   --dev $DEV \
   --reset-cache \
   --bundle-output "$BUNDLE_FILE" \
-  --assets-dest "$DEST"
+  --assets-dest "$DEST" \
+  $EXTRA_PACKAGER_ARGS
 
 if [[ $DEV != true && ! -f "$BUNDLE_FILE" ]]; then
   echo "error: File $BUNDLE_FILE does not exist. This must be a bug with" >&2