feature: Webhook message edit & delete functionality (#1753)

* feature: Webhook message edit & delete functionality

* PR fixes: Rename Edit* to Modify*; Add more detailed docstrings; Small validation fixes

* Fix spacing around docstrings

* Make ModifyWebhookMessageParams.Content Optional<string>

* Change the Webhook message edit functionality to use a object delegate method instead providing the all parameters

Co-authored-by: Desmont <desmont@users.noreply.github.com>

src/Discord.Net.Webhook/Entities/Messages/WebhookMessageProperties.cs

@@ -0,0 +1,26 @@
+using System.Collections.Generic;
+
+namespace Discord.Webhook
+{
+    /// <summary>
+    ///     Properties that are used to modify an Webhook message with the specified changes.
+    /// </summary>
+    public class WebhookMessageProperties
+    {
+        /// <summary>
+        ///     Gets or sets the content of the message.
+        /// </summary>
+        /// <remarks>
+        ///     This must be less than the constant defined by <see cref="DiscordConfig.MaxMessageSize"/>.
+        /// </remarks>
+        public Optional<string> Content { get; set; }
+        /// <summary>
+        ///     Gets or sets the embed array that the message should display.
+        /// </summary>
+        public Optional<IEnumerable<Embed>> Embeds { get; set; }
+        /// <summary>
+        ///     Gets or sets the allowed mentions of the message.
+        /// </summary>
+        public Optional<AllowedMentions> AllowedMentions { get; set; }
+    }
+}