invite method
void
invite()
Implementation
void invite(String callerName, String callerNumber, String destinationNumber,
String clientState, String callId, String telnyxSessionId,Map<String,String> customHeaders) async {
var sessionId = _selfId;
Session session = await _createSession(null,
peerId: "0", sessionId: sessionId, media: "audio");
_sessions[sessionId] = session;
_createOffer(session, "audio", callerName, callerNumber, destinationNumber,
clientState, callId, telnyxSessionId,customHeaders);
onCallStateChange?.call(session, CallState.CallStateInvite);
}