api: add slow mode

This adds the following property to ITextChannel
- SlowMode (int)

The SlowMode field defines the time in seconds users must wait between
sending messages; if zero, slow-mode is disabled.

Bots are not affected by slow-mode, and as such, no changes need to be
made to the REST client's internal ratelimiting. This is strictly a
read/write cosmetic property for bots.

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

@@ -45,7 +45,8 @@ namespace Discord.Rest
                 Position = args.Position,
                 CategoryId = args.CategoryId,
                 Topic = args.Topic,
-                IsNsfw = args.IsNsfw
+                IsNsfw = args.IsNsfw,
+                SlowMode = args.SlowMode,
             };
             return await client.ApiClient.ModifyGuildChannelAsync(channel.Id, apiArgs, options).ConfigureAwait(false);
         }