feature: Webhook message edit & delete functionality

src/Discord.Net.Rest/API/Rest/EditWebhookMessageParams.cs

@@ -0,0 +1,22 @@
+#pragma warning disable CS1591
+using Newtonsoft.Json;
+
+namespace Discord.API.Rest
+{
+    [JsonObject(MemberSerialization = MemberSerialization.OptIn)]
+    internal class EditWebhookMessageParams
+    {
+        [JsonProperty("content")]
+        public string Content { get; }
+
+        [JsonProperty("embeds")]
+        public Optional<Embed[]> Embeds { get; set; }
+        [JsonProperty("allowed_mentions")]
+        public Optional<AllowedMentions> AllowedMentions { get; set; }
+
+        public EditWebhookMessageParams(string content)
+        {
+            Content = content;
+        }
+    }
+}

src/Discord.Net.Rest/DiscordRestApiClient.cs

@@ -523,6 +523,44 @@ namespace Discord.API
             var ids = new BucketIds(webhookId: webhookId);
             return await SendJsonAsync<Message>("POST", () => $"webhooks/{webhookId}/{AuthToken}?wait=true", args, ids, clientBucket: ClientBucketType.SendEdit, options: options).ConfigureAwait(false);
         }
+
+        /// <exception cref="ArgumentOutOfRangeException">Message content is too long, length must be less or equal to <see cref="DiscordConfig.MaxMessageSize"/>.</exception>
+        /// <exception cref="InvalidOperationException">This operation may only be called with a <see cref="TokenType.Webhook"/> token.</exception>
+        public async Task EditWebhookMessageAsync(ulong webhookId, ulong messageId, EditWebhookMessageParams args, RequestOptions options = null)
+        {
+            if (AuthTokenType != TokenType.Webhook)
+                throw new InvalidOperationException($"This operation may only be called with a {nameof(TokenType.Webhook)} token.");
+
+            Preconditions.NotNull(args, nameof(args));
+            Preconditions.NotEqual(webhookId, 0, nameof(webhookId));
+            Preconditions.NotEqual(messageId, 0, nameof(messageId));
+            if (!args.Embeds.IsSpecified || args.Embeds.Value == null || args.Embeds.Value.Length == 0)
+                Preconditions.NotNullOrEmpty(args.Content, nameof(args.Content));
+
+            if (args.Content?.Length > DiscordConfig.MaxMessageSize)
+                throw new ArgumentException(message: $"Message content is too long, length must be less or equal to {DiscordConfig.MaxMessageSize}.", paramName: nameof(args.Content));
+            options = RequestOptions.CreateOrClone(options);
+
+            var ids = new BucketIds(webhookId: webhookId);
+            await SendJsonAsync<Message>("PATCH", () => $"webhooks/{webhookId}/{AuthToken}/messages/{messageId}", args, ids, clientBucket: ClientBucketType.SendEdit, options: options).ConfigureAwait(false);
+
+        }
+
+        /// <exception cref="InvalidOperationException">This operation may only be called with a <see cref="TokenType.Webhook"/> token.</exception>
+        public async Task DeleteWebhookMessageAsync(ulong webhookId, ulong messageId, RequestOptions options = null)
+        {
+            if (AuthTokenType != TokenType.Webhook)
+                throw new InvalidOperationException($"This operation may only be called with a {nameof(TokenType.Webhook)} token.");
+
+            Preconditions.NotEqual(webhookId, 0, nameof(webhookId));
+            Preconditions.NotEqual(messageId, 0, nameof(messageId));
+
+            options = RequestOptions.CreateOrClone(options);
+
+            var ids = new BucketIds(webhookId: webhookId);
+            await SendAsync("DELETE", () => $"webhooks/{webhookId}/{AuthToken}/messages/{messageId}", ids, options: options).ConfigureAwait(false);
+        }
+
         /// <exception cref="ArgumentOutOfRangeException">Message content is too long, length must be less or equal to <see cref="DiscordConfig.MaxMessageSize"/>.</exception>
         public async Task<Message> UploadFileAsync(ulong channelId, UploadFileParams args, RequestOptions options = null)
         {

src/Discord.Net.Webhook/DiscordWebhookClient.cs

@@ -91,6 +91,15 @@ namespace Discord.Webhook
             string username = null, string avatarUrl = null, RequestOptions options = null, AllowedMentions allowedMentions = null)
             => WebhookClientHelper.SendMessageAsync(this, text, isTTS, embeds, username, avatarUrl, allowedMentions, options);
 
+        /// <summary> Edits a message posted using this webhook. </summary>
+        public Task EditMessageAsync(ulong messageId, string text = null, IEnumerable<Embed> embeds = null,
+            AllowedMentions allowedMentions = null, RequestOptions options = null)
+            => WebhookClientHelper.EditMessageAsync(this, messageId, text, embeds, allowedMentions, options);
+
+        /// <summary> Deletes a message posted using this webhook. </summary>
+        public Task DeleteMessageAsync(ulong messageId, RequestOptions options = null)
+            => WebhookClientHelper.DeleteMessageAsync(this, messageId, options);
+
         /// <summary> Sends a message to the channel for this webhook with an attachment. </summary>
         /// <returns> Returns the ID of the created message. </returns>
         public Task<ulong> SendFileAsync(string filePath, string text, bool isTTS = false,

src/Discord.Net.Webhook/WebhookClientHelper.cs

@@ -36,6 +36,21 @@ namespace Discord.Webhook
             var model = await client.ApiClient.CreateWebhookMessageAsync(client.Webhook.Id, args, options: options).ConfigureAwait(false);
             return model.Id;
         }
+        public static async Task EditMessageAsync(DiscordWebhookClient client, ulong messageId, string text, IEnumerable<Embed> embeds, 
+            AllowedMentions allowedMentions, RequestOptions options)
+        {
+            var args = new EditWebhookMessageParams(text);
+            if (embeds != null)
+                args.Embeds = embeds.Select(x => x.ToModel()).ToArray();
+            if (allowedMentions != null)
+                args.AllowedMentions = allowedMentions.ToModel();
+
+            await client.ApiClient.EditWebhookMessageAsync(client.Webhook.Id, messageId, args, options: options).ConfigureAwait(false);
+        }
+        public static async Task DeleteMessageAsync(DiscordWebhookClient client, ulong messageId, RequestOptions options)
+        {
+            await client.ApiClient.DeleteWebhookMessageAsync(client.Webhook.Id, messageId, options).ConfigureAwait(false);
+        }
         public static async Task<ulong> SendFileAsync(DiscordWebhookClient client, string filePath, string text, bool isTTS, 
             IEnumerable<Embed> embeds, string username, string avatarUrl, AllowedMentions allowedMentions, RequestOptions options, bool isSpoiler)
         {