Hello
I've noticed that symfony/firebase-notifier has no support for the data field to be sent to the Firebase API server. It only sends the to and notification fields ( https://firebase.google.com/docs/cloud-messaging/http-server-ref ).
I need that field for a project I am currently working on.
Is there a particular reason for this not being implemented?
I would like to open a PR for setting up this field in the payload as well.

What needs to be done is add the data field in the FirebaseOptions class as well.