add v4 option 108: v6only-preferred

There's an I-D being evaluated by IESG right now: https://datatracker.ietf.org/doc/draft-ietf-dhc-v6only/

It went through the early allocation phase and got 108 code assigned. We should update Kea to handle this option.

Note: This is DHCPv4 option, not v6. It informs dual-stack devices that v6-only is the preferred way to go, but it's delivered using DHCPv4.