RFC4014, RFC7037: Radius option definitions

The RadiusDesign calls for an optional mechanism that will query the Radius server about specific client. Typically this functionality has been done by a relay, which then inserted Radius options into DHCP message before forwarding it to the server.

Kea should be able to understand such options. See RFC4014 (v4) and RFC7037 (v6) for details. Kea should be able to represent radius attributes as sub-options, so general mechanisms, like client classification could be used.

This ticket calls for option definitions only. No special handling logic should be implemented.