[Feature] Age restricted (NSFW) application commands support (#2531)

* add `nsfw` to data model & internal methods; add missing property

* add `nsfw` prop to command builders

* add `NsfwCommandAttribute` to Interaction Framework

* working state

* docs?

src/Discord.Net.WebSocket/Entities/Interaction/SocketBaseCommand/SocketApplicationCommand.cs

@@ -39,6 +39,9 @@ namespace Discord.WebSocket
         /// <inheritdoc/>
         public bool IsEnabledInDm { get; private set; }
 
+        /// <inheritdoc/>
+        public bool IsNsfw { get; private set; }
+
         /// <inheritdoc/>
         public GuildPermissions DefaultMemberPermissions { get; private set; }
 
@@ -130,6 +133,7 @@ namespace Discord.WebSocket
 
             IsEnabledInDm = model.DmPermission.GetValueOrDefault(true).GetValueOrDefault(true);
             DefaultMemberPermissions = new GuildPermissions((ulong)model.DefaultMemberPermission.GetValueOrDefault(0).GetValueOrDefault(0));
+            IsNsfw = model.Nsfw.GetValueOrDefault(false).GetValueOrDefault(false);
         }
 
         /// <inheritdoc/>