#358 support for additional apns fields in the payload

[Anonymous]

Originally created by: RohitGupta31

APNS payload - need support for 'mutable-content' & 'alert' fields in the aps dictionary.

Alert - Sets the alert to be specified as a String to display as the message text of the alert or banner.
https://developer.apple.com/library/archive/documentation/NetworkingInternet/Conceptual/RemoteNotificationsPG/PayloadKeyReference.html#//apple_ref/doc/uid/TP40008194-CH17-SW1

Mutable-Content Sets the extension notification type 'mutable-content' so that the client can let the OS know that they are not using the standard JSON format, rather a customised one which we will be parsed and let the OS know what contents needs to be shown. https://developer.apple.com/documentation/usernotifications/unnotificationserviceextension#overview