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.WebSocket/Entities/Channels/SocketVoiceChannel.cs

@@ -82,6 +82,12 @@ namespace Discord.WebSocket
         public async Task DisconnectAsync()
             => await Guild.DisconnectAudioAsync();
 
+        /// <inheritdoc />
+        public async Task ModifyAsync(Action<AudioChannelProperties> func, RequestOptions options = null)
+        {
+            await Guild.ModifyAudioAsync(Id, func, options).ConfigureAwait(false);
+        }
+
         /// <inheritdoc />
         public override SocketGuildUser GetUser(ulong id)
         {