feat: add method to get fcm token

[yousrasd]

• Implement a method to retrieve the FCM token upon request. I require this functionality in my app as I only need to obtain the FCM token once when the app is opened. I prefer not to use the onRegister method as it triggers continuously.

Goes with the following PR in react-native-push-notification/ios: react-native-push-notification/ios#410

[jedabero]

Why not use onRegister passed to the configuration options?

PushNotification.configure({
    requestPermissions: true,
    onRegister: ({ os, token }) => {
      // use token
    },
}):