add a buffer for the data received from the server

[jeanrocco]

In this answer: https://github.com/alejho/Arduino-Telnet-Client/issues/9#issuecomment-528912914 you said " I'm sorry this library doesn't support answer validation from the server...I suppose you have to store the answer in a buffer and the parse it lookin for the information you need! "

Could you indeed provide a call to store the answer (payload) in a buffer which we can then analyze ...

It is not easy for my level of coding ! thank's jrb