Add support for specifying interruption-level

[mman]

When sending a push notification to the iOS 15 app, a notification payload can specify interruption-level value that is later populated into UNNotificationContent when push notification is received and can specify importance of a message.

WWCD Video: https://developer.apple.com/videos/play/wwdc2021/10091/
APNS reference: https://developer.apple.com/documentation/usernotifications/setting_up_a_remote_notification_server/generating_a_remote_notification

Looks like a good and simple starter PR where interruption-level is basically populated through the same way mutable-content is.

Will require changes to underlying https://github.com/parse-community/node-apn module.

[mman]

This is obviously available for a very long time, closing.