Custom DHCP option processing

Implemented functionality to request a custom DHCP option and process the response in a callback.

This allows an application to request additional options from the DHCP server (such as the address of an NTP server or TFTP server) and receive the response. List of DHCP options is maintained by IANA:
https://www.iana.org/assignments/bootp-dhcp-parameters/bootp-dhcp-parameters.xhtml

The currently implementation only support a single option and callback but the design of the API is designed to allow additional options for a future development.

There is an example of using this functionality to receive a list of NTP servers here:
https://github.com/njh/EtherCardClock