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.WebSocket/API/Gateway/WebhookUpdateEvent.cs

@@ -0,0 +1,13 @@
+#pragma warning disable CS1591
+using Newtonsoft.Json;
+
+namespace Discord.API.Gateway
+{
+    internal class WebhookUpdateEvent
+    {
+        [JsonProperty("guild_id")]
+        public ulong GuildId { get; set; }
+        [JsonProperty("channel_id")]
+        public ulong ChannelId { get; set; }
+    }
+}