Support FCM analytics_label

[strayer]

Firebase now supports an analytics label in FCM, which can be really useful in combination with Big Query and such.

This is only supported in the current API as far as I can see, so supporting this would probably require moving to that one, making this a little more complicated.

https://firebase.google.com/docs/cloud-messaging/understand-delivery#adding_analytics_labels_to_messages https://firebase.google.com/docs/reference/fcm/rest/v1/projects.messages#fcmoptions