feat: add support to subscribe to/unsubscribe from a Firebase topic

[yousrasd]

The main lib https://github.com/zo0r/react-native-push-notification does offer an option to subscribe to a Firebase topic on Android. But this ios lib doesn't.

I need this functionality on ios too. Therefore, I made the necessary changes in this pull request to expose a method that allows subscribing to or unsubscribing from a Firebase topic.

Note: I set the method signature as 'fire and forget' to align with the method contract in the main repository.

[yousrasd]

@Naturalclar It would be greatly appreciated if you could review this pull request. I genuinely need these changes in my project as soon as possible.