[Feature] Allow switching voice channels without disconnecting (external clients only) (#2866)

* eh well it should work

* fix issues

* make sure an external client is used

test/Discord.Net.Tests.Unit/MockedEntities/MockedGroupChannel.cs

@@ -18,7 +18,7 @@ namespace Discord
 
         public string RTCRegion => throw new NotImplementedException();
 
-        public Task<IAudioClient> ConnectAsync(bool selfDeaf = false, bool selfMute = false, bool external = false)
+        public Task<IAudioClient> ConnectAsync(bool selfDeaf = false, bool selfMute = false, bool external = false, bool disconnect = true)
         {
             throw new NotImplementedException();
         }