Allow users to bypass builtin voice connection management

[foxbot]

By allowing users to bypass discordgo's builtin voice connection management, they can use third-party voice solutions such as Lavalink

The only changes necessary to the lib would be adding a public function to discordgo.Session to send an op4 to Discord to initiate the voice handshake and return a VOICE_SERVER_UPDATE. The library already exposes an event for VOICE_SERVER_UPDATE with the proper fields; however, the library also intercepts this event and uses it to complete its own voice connection.

This would be a pretty simple change, and should only end up being something like

func (s *Session) ChannelVoiceJoinManual(gID, cID string, mute, deaf bool) (err error) {
	// ripped from https://github.com/bwmarrin/discordgo/blob/master/wsapi.go#L598
	
	// Send the request to Discord that we want to join the voice channel
	data := voiceChannelJoinOp{4, voiceChannelJoinData{&gID, &cID, mute, deaf}}
	s.wsMutex.Lock()
	err = s.wsConn.WriteJSON(data)
	s.wsMutex.Unlock()
	if err != nil {
		return
	}
}