[Feature] Avatar decorations support (#2782)

* yup

* nullable...

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

src/Discord.Net.Core/CDN.cs

@@ -257,5 +257,15 @@ namespace Discord
                 _ => throw new ArgumentException(nameof(format)),
             };
         }
+
+        /// <summary>
+        ///     Gets an avatar decoration url based off the hash.
+        /// </summary>
+        /// <param name="avatarDecorationHash">The hash of the avatar decoraition.</param>
+        /// <returns>
+        ///     A URL to the avatar decoration.
+        /// </returns>
+        public static string GetAvatarDecorationUrl(string avatarDecorationHash)
+            => $"{DiscordConfig.CDNUrl}avatar-decoration-presets/{avatarDecorationHash}.png";
     }
 }

src/Discord.Net.Core/Entities/Users/IUser.cs

@@ -95,6 +95,22 @@ namespace Discord
         /// </remarks>
         string GlobalName { get; }
 
+        /// <summary>
+        ///     Gets the hash of the avatar decoration.
+        /// </summary>
+        /// <remarks>
+        ///     <see langword="null"/> if the user has no avatar decoration set.
+        /// </remarks>
+        string AvatarDecorationHash { get; }
+
+        /// <summary>
+        ///     Gets the id of the avatar decoration's SKU.
+        /// </summary>
+        /// <remarks>
+        ///     <see langword="null"/> if the user has no avatar decoration set.
+        /// </remarks>
+        ulong? AvatarDecorationSkuId { get; }
+
         /// <summary>
         ///     Creates the direct message channel of this user.
         /// </summary>
@@ -120,5 +136,14 @@ namespace Discord
         ///     contains the DM channel associated with this user.
         /// </returns>
         Task<IDMChannel> CreateDMChannelAsync(RequestOptions options = null);
+
+
+        /// <summary>
+        ///     Gets the URL for user's avatar decoration.
+        /// </summary>
+        /// <remarks>
+        ///     <see langword="null"/> if the user has no avatar decoration set.
+        /// </remarks>
+        string GetAvatarDecorationUrl();
     }
 }