Support NTP option 42 in DHCP (as well as other options, possibly)

[Rocco83]

Hi,

I'm likely sure that i'll be blasted by a RTFM but... i was not able to find out why the Ethernet class is not supporting the NTP/Time options (eg option 42) to allow the NTP setup in a dynamic way. I believe that the approach is KISS and memory footprint, but i would like to understand it in detail.

Footnote: In worst case (option will be not supported), I believe that some lines should detail in README why options A-B-C are not supported... so that's why i'm opening an issue and not a port in the forum.

This was already referenced in the forum quite time ago, with a way to take out only the first NTP entry. https://forum.arduino.cc/t/ethernet-shield-get-ntp-server-via-dhcp/256676/2 with a working workaround:

1. comment out option 42: https://github.com/arduino-libraries/Ethernet/blob/75a3c37b5e513305b82e926ca6a4f8190f536c9d/src/Dhcp.h#L97
2. add the following snippet (i would say nearby https://github.com/arduino-libraries/Ethernet/blob/master/src/Dhcp.cpp#L327)

                case ntpServers :
                    opt_len = _dhcpUdpSocket.read();
                    _dhcpUdpSocket.read(_dhcpNtpServerIp, 4);
                    for (int i = 0; i < opt_len-4; i++)
                    {
                        _dhcpUdpSocket.read();
                    }
                    break;

Thanks!