Add support for collapse_id for iOS 10 notifications

[mman]

HTTP/2 header field collapse_id is used to update push notifications instead of generating new ones on the iOS 10.

iOS 10 uses the UNNotificationRequest.identifier (https://developer.apple.com/reference/usernotifications/unnotificationrequest/1649634-identifier) to achieve the same functionality for local notifications generated on device.

OneSignal has recently added support for collapse_id in their REST API (https://documentation.onesignal.com/reference#create-notification) so it's possible now to implement the same in parse-server-onesignal-push-adapter.