Add SyncPermissionsAsync to Sync Child Channels with its Parent (#1159)

* Initial implementation

* Adjust according to comments

See: 6e76b45713 (diff-58466c35787d448266d026692e467baa)

src/Discord.Net.Rest/Entities/Channels/RestVoiceChannel.cs

@@ -57,6 +57,8 @@ namespace Discord.Rest
         /// </returns>
         public Task<ICategoryChannel> GetCategoryAsync(RequestOptions options = null)
             => ChannelHelper.GetCategoryAsync(this, Discord, options);
+        public Task SyncPermissionsAsync(RequestOptions options = null)
+            => ChannelHelper.SyncPermissionsAsync(this, Discord, options);
 
         private string DebuggerDisplay => $"{Name} ({Id}, Voice)";