Add token validation service

Added an MR of patch #2 to make this easier to review.

A related question to this topic: What do you recommend for regularly checking whether the tokens already saved are still valid?

Would a good option be, for example, a cronjob that tests all stored tokens with this service every week/month and deletes them from the database if they are negative?

@see https://firebase.google.com/docs/cloud-messaging/manage-tokens#update-to...

@handkerchief This module currently has no feature for managing whether tokens are still valid. That would be a useful feature to have, but it should probably be made as a feature request in a separate issue.

Also, for what it's worth, this module is not actively maintained, so hopefully you're able to contribute the code yourself.

I switched to the project firebase_php → . It has validation features as well as the modern way of API as google is discontinuing the old one soon.