Allow external VoiceChannel client API implementation (#1057)

* ConnectAsync(bool external) API implemented

This allows developers to handle the AudioClient externally (for example with Lavalink)

* Modify ConnectAsync API and add DisconnectAsync to IAudioChannel

* Update summary message on IAudioChannel.DisconnectAsync()

* Review changes:
- Fix `if (!external)` styling
- Remove unecessary ConnectAsync overload

* SocketVoiceChannel overload update

* Update IAudioChannel.ConnectAsync()
- Remove ConfigAction from parameters
- Add SelfDeafen/SelfMute to parameters

* Remove SocketVoiceChannel.ConnectAsync() default params (Inherit)

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

@@ -1,4 +1,4 @@
-using Discord.Audio;
+using Discord.Audio;
 using System;
 using System.Threading.Tasks;
 
@@ -7,6 +7,9 @@ namespace Discord
     public interface IAudioChannel : IChannel
     {
         /// <summary> Connects to this audio channel. </summary>
-        Task<IAudioClient> ConnectAsync(Action<IAudioClient> configAction = null);
+        Task<IAudioClient> ConnectAsync(bool selfDeaf = false, bool selfMute = false, bool external = false);
+
+        /// <summary> Disconnects from this audio channel. </summary>
+        Task DisconnectAsync();
     }
 }