sd-dhcp6-client: Add IA Address option parsing

sd-dhcp6-client: Add IA Address option parsing

Add functionality to parse DHCPv6 Identity Association for
Non-temporary (IA_NA) and Temporary Addresses (IA_TA) options.
Both of them contain one or more IA Address (IAADDR) options
and optinally a status code option. Only the IA_NA option
contains lease lifetimes. See RFC 3315, sections 22.4., 22.5.,
22.6., 22.13. and appendix B. for details. If the lease
timeouts are not set, use the ones recommended for servers in
section 22.4.

Factor out common code in the form of an option header parsing
helper function.