Add support for specifying target-content-id

[mman]

When sending a push notification to the iOS 13 multi window app, a notification payload can specify target-content-id value that is later populated into UNNotificationContent when push notification is received and can later be used to target what app window should be brought forward when the notification is tapped.

WWDC video: https://developer.apple.com/videos/play/wwdc2019/259/
APN reference: https://developer.apple.com/documentation/usernotifications/setting_up_a_remote_notification_server/generating_a_remote_notification
UNNotificationContent: https://developer.apple.com/documentation/usernotifications/unnotificationcontent/3235764-targetcontentidentifier

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

[funkenstrahlen]

This might require adding support for this property to node-apn before this can be used here.

[parse-github-assistant]

The label type:feature cannot be used in combination with type:improvement.

[mman]

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