Add missing REST Webhook implemenation (#843)

* Add Webhook API models, REST implementation, and Socket bridges.

* Remove token overrides from REST.

Leaving that as a Webhook package only feature.

* Add Webhook API models, REST implementation, and Socket bridges.

* Remove token overrides from REST.

Leaving that as a Webhook package only feature.

* Webhook core implementation.

* Webhook REST implementation.

* Webhook client implementation.

* Add channel bucket id.

src/Discord.Net.Rest/Entities/Guilds/RestGuild.cs

@@ -257,6 +257,12 @@ namespace Discord.Rest
         public Task<int> PruneUsersAsync(int days = 30, bool simulate = false, RequestOptions options = null)
             => GuildHelper.PruneUsersAsync(this, Discord, days, simulate, options);
 
+        //Webhooks
+        public Task<RestWebhook> GetWebhookAsync(ulong id, RequestOptions options = null)
+            => GuildHelper.GetWebhookAsync(this, Discord, id, options);
+        public Task<IReadOnlyCollection<RestWebhook>> GetWebhooksAsync(RequestOptions options = null)
+            => GuildHelper.GetWebhooksAsync(this, Discord, options);
+
         public override string ToString() => Name;
         private string DebuggerDisplay => $"{Name} ({Id})";
 
@@ -396,5 +402,10 @@ namespace Discord.Rest
                 return ImmutableArray.Create<IGuildUser>();
         }
         Task IGuild.DownloadUsersAsync() { throw new NotSupportedException(); }
+
+        async Task<IWebhook> IGuild.GetWebhookAsync(ulong id, RequestOptions options)
+            => await GetWebhookAsync(id, options);
+        async Task<IReadOnlyCollection<IWebhook>> IGuild.GetWebhooksAsync(RequestOptions options)
+            => await GetWebhooksAsync(options);
     }
 }