feature: add ChannelVoiceJoinManual

This resolves #577.

ChannelVoiceJoinManual is a wrapper over sending an OP4 to Discord, for
initiating a voice server connection. The library's builtin voice
connection management locks/maps are skipped, and the library will not
attempt to manage this voice connection.

Users are expected to hook the VoiceServerUpdate event and forward the
data to an outside voice manager, such as Lavalink.

wsapi.go

@@ -623,6 +623,30 @@ func (s *Session) ChannelVoiceJoin(gID, cID string, mute, deaf bool) (voice *Voi
 	return
 }
 
+// ChannelVoiceJoinManual initiates a voice session to a voice channel, but does not complete it.
+//
+// This should only be used when the VoiceServerUpdate will be intercepted and used elsewhere.
+//
+//    gID     : Guild ID of the channel to join.
+//    cID     : Channel ID of the channel to join.
+//    mute    : If true, you will be set to muted upon joining.
+//    deaf    : If true, you will be set to deafened upon joining.
+func (s *Session) ChannelVoiceJoinManual(gID, cID string, mute, deaf bool) (err error) {
+
+	s.log(LogInformational, "called")
+
+	// 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
+	}
+
+	return
+}
+
 // onVoiceStateUpdate handles Voice State Update events on the data websocket.
 func (s *Session) onVoiceStateUpdate(st *VoiceStateUpdate) {