[Feature] Avatar decorations support (#2782)

* yup

* nullable...

* add props & methos to `GuildUser` & `IUser`

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

@@ -22,6 +22,9 @@ namespace Discord.API
         [JsonProperty("global_name")]
         public Optional<string> GlobalName { get; set; }
 
+        [JsonProperty("avatar_decoration_data")]
+        public Optional<AvatarDecorationData> AvatarDecoration { get; set; }
+
         //CurrentUser
         [JsonProperty("verified")]
         public Optional<bool> Verified { get; set; }