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/Webhooks/RestWebhook.cs

@@ -0,0 +1,91 @@
+using System;
+using System.Diagnostics;
+using System.Threading.Tasks;
+using Model = Discord.API.Webhook;
+
+namespace Discord.Rest
+{
+    [DebuggerDisplay(@"{DebuggerDisplay,nq}")]
+    public class RestWebhook : RestEntity<ulong>, IWebhook, IUpdateable
+    {
+        internal IGuild Guild { get; private set; }
+        internal ITextChannel Channel { get; private set; }
+
+        public ulong ChannelId { get; }
+        public string Token { get; }
+
+        public string Name { get; private set; }
+        public string AvatarId { get; private set; }
+        public ulong? GuildId { get; private set; }
+        public IUser Creator { get; private set; }
+
+        public DateTimeOffset CreatedAt => SnowflakeUtils.FromSnowflake(Id);
+
+        internal RestWebhook(BaseDiscordClient discord, IGuild guild, ulong id, string token, ulong channelId)
+            : base(discord, id)
+        {
+            Guild = guild;
+            Token = token;
+            ChannelId = channelId;
+        }
+        internal RestWebhook(BaseDiscordClient discord, ITextChannel channel, ulong id, string token, ulong channelId)
+            : this(discord, channel.Guild, id, token, channelId)
+        {
+            Channel = channel;
+        }
+
+        internal static RestWebhook Create(BaseDiscordClient discord, IGuild guild, Model model)
+        {
+            var entity = new RestWebhook(discord, guild, model.Id, model.Token, model.ChannelId);
+            entity.Update(model);
+            return entity;
+        }
+        internal static RestWebhook Create(BaseDiscordClient discord, ITextChannel channel, Model model)
+        {
+            var entity = new RestWebhook(discord, channel, model.Id, model.Token, model.ChannelId);
+            entity.Update(model);
+            return entity;
+        }
+
+        internal void Update(Model model)
+        {
+            if (model.Avatar.IsSpecified)
+                AvatarId = model.Avatar.Value;
+            if (model.Creator.IsSpecified)
+                Creator = RestUser.Create(Discord, model.Creator.Value);
+            if (model.GuildId.IsSpecified)
+                GuildId = model.GuildId.Value;
+            if (model.Name.IsSpecified)
+                Name = model.Name.Value;
+        }
+
+        public async Task UpdateAsync(RequestOptions options = null)
+        {
+            var model = await Discord.ApiClient.GetWebhookAsync(Id, options).ConfigureAwait(false);
+            Update(model);
+        }
+
+        public string GetAvatarUrl(ImageFormat format = ImageFormat.Auto, ushort size = 128)
+           => CDN.GetUserAvatarUrl(Id, AvatarId, size, format);
+
+        public async Task ModifyAsync(Action<WebhookProperties> func, RequestOptions options = null)
+        {
+            var model = await WebhookHelper.ModifyAsync(this, Discord, func, options).ConfigureAwait(false);
+            Update(model);
+        }
+
+        public Task DeleteAsync(RequestOptions options = null)
+            => WebhookHelper.DeleteAsync(this, Discord, options);
+
+        public override string ToString() => $"Webhook: {Name}:{Id}";
+        private string DebuggerDisplay => $"Webhook: {Name} ({Id})";
+
+        //IWebhook
+        IGuild IWebhook.Guild 
+            => Guild ?? throw new InvalidOperationException("Unable to return this entity's parent unless it was fetched through that object.");
+        ITextChannel IWebhook.Channel 
+            => Channel ?? throw new InvalidOperationException("Unable to return this entity's parent unless it was fetched through that object.");
+        Task IWebhook.ModifyAsync(Action<WebhookProperties> func, RequestOptions options)
+            => ModifyAsync(func, options);
+    }
+}