Add extended header support

[zi6xuan]

Add extended header support, in order to carry custom information when calling

This is an example code to get the header content

  static Map<String, dynamic> getHeaders(Call call) {
    // IncomingRequest/OutgoingRequest
    var request = call.session.request;
    if (request is IncomingRequest) {
      return request.headers;
    } else if (request is OutgoingRequest) {
      return GUIUtils.parseHeaders(request.extraHeaders);
    }
    return {};
  }

[cloudwebrtc]

LGTM