[Feature] Add purchase notification (#2942)

src/Discord.Net.Rest/API/Common/Message.cs

@@ -105,6 +105,9 @@ internal class Message
     [JsonProperty("poll")]
     public Optional<Poll> Poll { get; set; }
 
+    [JsonProperty("purchase_notification")]
+    public Optional<MessagePurchaseNotification> PurchaseNotification { get; set; }
+  
     [JsonProperty("call")]
     public Optional<MessageCallData> Call { get; set; }
 }