pltlg

Added http header support. Example: ```dart Map headers = {'Authorization': 'Bearer xyz'}; ros = Ros(url: 'wss://host.com', headers: headers); ``` Works with IOWebSocketChannel. HtmlWebSocketChannel support needed!

I think it would be beneficial to support http headers. ```dart Map headers = {'Authorization': 'Bearer xyz'}; ros = Ros(url: 'ws://host.com', headers: headers); ```