[Feature] Avatar decorations support (#2782)

* yup

* nullable...

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

src/Discord.Net.Rest/Entities/Users/RestUser.cs

@@ -4,6 +4,7 @@ using System.Collections.Immutable;
 using System.Diagnostics;
 using System.Globalization;
 using System.Threading.Tasks;
+
 using EventUserModel = Discord.API.GuildScheduledEventUser;
 using Model = Discord.API.User;
 
@@ -50,6 +51,13 @@ namespace Discord.Rest
         /// <inheritdoc />
         public virtual bool IsWebhook => false;
 
+        /// <inheritdoc />
+        public string AvatarDecorationHash { get; private set; }
+
+        /// <inheritdoc />
+        public ulong? AvatarDecorationSkuId { get; private set; }
+
+
         internal RestUser(BaseDiscordClient discord, ulong id)
             : base(discord, id)
         {
@@ -96,6 +104,11 @@ namespace Discord.Rest
                 PublicFlags = model.PublicFlags.Value;
             if (model.GlobalName.IsSpecified)
                 GlobalName = model.GlobalName.Value;
+            if (model.AvatarDecoration is { IsSpecified: true, Value: not null })
+            {
+                AvatarDecorationHash = model.AvatarDecoration.Value?.Asset;
+                AvatarDecorationSkuId = model.AvatarDecoration.Value?.SkuId;
+            }
         }
 
         /// <inheritdoc />
@@ -129,6 +142,12 @@ namespace Discord.Rest
                 ? CDN.GetDefaultUserAvatarUrl(DiscriminatorValue)
                 : CDN.GetDefaultUserAvatarUrl(Id);
 
+        /// <inheritdoc />
+        public string GetAvatarDecorationUrl()
+            => AvatarDecorationHash is not null
+                ? CDN.GetAvatarDecorationUrl(AvatarDecorationHash)
+                : null;
+
         /// <summary>
         ///     Gets the Username#Discriminator of the user.
         /// </summary>