Allow clients to send 'Gateway Voice State Update' command (#1888)

* Expose SendVoiceStateUpdateAsync API to clients

Fixes #1882

* Revert "Expose SendVoiceStateUpdateAsync API to clients"

This reverts commit 1a11cae7

* Add IAudioChannel.ModifyAsync API

* fix NRE with request options

Co-authored-by: Quin Lynch <49576606+quinchs@users.noreply.github.com>
Co-authored-by: quin lynch <lynchquin@gmail.com>

src/Discord.Net.Core/Entities/Channels/AudioChannelProperties.cs

@@ -0,0 +1,18 @@
+namespace Discord
+{
+    /// <summary>
+    ///     Provides properties that are used to modify an <see cref="IAudioChannel" /> with the specified changes.
+    /// </summary>
+    public class AudioChannelProperties
+    {
+        /// <summary>
+        ///     Sets whether the user should be muted.
+        /// </summary>
+        public Optional<bool> SelfMute { get; set; }
+
+        /// <summary>
+        ///     Sets whether the user should be deafened.
+        /// </summary>
+        public Optional<bool> SelfDeaf { get; set; }
+    }
+}

src/Discord.Net.Core/Entities/Channels/IAudioChannel.cs

@@ -1,4 +1,5 @@
 using Discord.Audio;
+using System;
 using System.Threading.Tasks;
 
 namespace Discord
@@ -27,5 +28,16 @@ namespace Discord
         ///     A task representing the asynchronous operation for disconnecting from the audio channel.
         /// </returns>
         Task DisconnectAsync();
+
+        /// <summary>
+        ///     Modifies this audio channel.
+        /// </summary>
+        /// <param name="func">The properties to modify the channel with.</param>
+        /// <param name="options">The options to be used when sending the request.</param>
+        /// <returns>
+        ///     A task that represents the asynchronous modification operation.
+        /// </returns>
+        /// <seealso cref="AudioChannelProperties"/>
+        Task ModifyAsync(Action<AudioChannelProperties> func, RequestOptions options = null);
     }
 }