[Feature] Default thread ratelimit in guild text channels (#2622)

* initial commit

* mocked bruh

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

@@ -65,6 +65,7 @@ namespace Discord.Rest
                         Deny = overwrite.Permissions.DenyValue.ToString()
                     }).ToArray()
                     : Optional.Create<API.Overwrite[]>(),
+                DefaultSlowModeInterval = args.DefaultSlowModeInterval
             };
             return await client.ApiClient.ModifyGuildChannelAsync(channel.Id, apiArgs, options).ConfigureAwait(false);
         }