Adds apns setter

[grahac]

adds a set_apns call. I am totally fine with you doing it a different way but this works!!!!

[hpopp]

I appreciate you using the package!

So at first glance, this is functionally no different from:

notification
|> Map.put(:apns, apns)

I'm open to the idea of well-named setters for FCM notifications, but unlike APNS.Notification, they wouldn't be doing much (except maybe type validations). Is there interest in having them? We'd need to have them defined for all of the FCM notification attributes, along with proper typespecs and docs.

[grahac]

Well duh. For some reason I I had it in my head that you couldn't just set the Map.put externally. My suggestion is just to show some examples of this in the readme and call it a day. Changing my source code now too :)