[Feature Request]: Add `apns-unique-id` in response

[eryue0220]

Currently the response only return the notification payload object. Could add a new param apns-unique-id to help debug, which is in the response header?

{
  "headers": {
    "apns-id": "ACDFB3EB-105B-D33C-7F29-8D09A01CC4EF",
    "apns-unique-id": "4084a6e4-9ea8-6d2a-ca49-9a344a152da7"
  }
}

[AndrewBarba]

Working on this for v13: https://github.com/AndrewBarba/apns2/pull/99