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.Rest/Entities/Channels/RestGroupChannel.cs

@@ -211,6 +211,7 @@ namespace Discord.Rest
         /// <exception cref="NotSupportedException">Connecting to a group channel is not supported.</exception>
         Task<IAudioClient> IAudioChannel.ConnectAsync(bool selfDeaf, bool selfMute, bool external) { throw new NotSupportedException(); }
         Task IAudioChannel.DisconnectAsync() { throw new NotSupportedException(); }
+        Task IAudioChannel.ModifyAsync(Action<AudioChannelProperties> func, RequestOptions options) { throw new NotSupportedException(); }
         #endregion
 
         #region IChannel        

src/Discord.Net.Rest/Entities/Channels/RestVoiceChannel.cs

@@ -95,6 +95,7 @@ namespace Discord.Rest
         /// <exception cref="NotSupportedException">Connecting to a REST-based channel is not supported.</exception>
         Task<IAudioClient> IAudioChannel.ConnectAsync(bool selfDeaf, bool selfMute, bool external) { throw new NotSupportedException(); }
         Task IAudioChannel.DisconnectAsync() { throw new NotSupportedException(); }
+        Task IAudioChannel.ModifyAsync(Action<AudioChannelProperties> func, RequestOptions options) { throw new NotSupportedException(); }
         #endregion
 
         #region IGuildChannel