Add JsonProperty attribute to Gifter property

[mohamedyassine-benmiled]

Fix read gifter on New Subscription event

[mohamedyassine-benmiled]

This edit was made because the webhook payload when a new subscription arrives is as follows

{
  "broadcaster": {
    "is_anonymous": false,
    "user_id": 58951189,
    "username": "AimonM",
    "is_verified": false,
    "profile_picture": "",
    "channel_slug": "aimonm",
    "identity": null
  },
  "subscriber": {
    "is_anonymous": false,
    "user_id": 55783,
    "username": "kaysharp",
    "is_verified": false,
    "profile_picture": "https://dbxmjjzl5pc1g.cloudfront.net/68417caf-7cdd-43e3-8a65-c6d605e1b881/images/user-profile-pic.png",
    "channel_slug": "kaysharp",
    "identity": null
  },
  "duration": 1,
  "created_at": "2025-10-09T16:18:40.927753489Z",
  "expires_at": "2025-11-16T16:18:40.916366187Z"
}

[Bukk94]

Thank you for your contribution. This was indeed an oversight on my part.
I'll write some tests to ensure all events are properly populated :)