[NEW] Send 'mutable-content = 1' in APN payload

This will allow us to support end-to-end encrypted notifications & also remove markdown in the client side.

Documentation:

https://developer.apple.com/library/archive/documentation/NetworkingInternet/Conceptual/RemoteNotificationsPG/ModifyingNotifications.html

https://developer.apple.com/documentation/usernotifications/modifying_content_in_newly_delivered_notifications

Example payload:

{
   “aps” : {
      “category” : “SECRET”,
      “mutable-content” : 1,
      “alert” : {
         “title” : “Secret Message!”,
         “body”  : “(Encrypted)”
     },
   },
   “ENCRYPTED_DATA” : “Salted__·öîQÊ$UDì_¶Ù∞è	Ω^¬%gq∞NÿÒQùw”
}

This will also require work in https://github.com/RocketChat/push & https://github.com/RocketChat/pushgateway

Start with the Apple notification-modification documentation linked in the issue, then inspect the APN payload path in Rocket.Chat and the related Rocket.Chat/push and Rocket.Chat/pushgateway repositories. Done means the payload includes mutable-content = 1 and the dependent encrypted-notification work is supported across the referenced services.