net: dhcpv6: Add Zephyr DHCPv6 client

Add a DHCPv6 client implementation for Zephyr (RFC 8415).

The implementation allows to request IPv6 address and/or prefix from the
DHCPv6 server, and for now supports only the mandatory set of DHCPv6
options needed to achieve this. Currently the implementation supports
the following scenarios:
 * Requesting new IPv6 address/prefix with Solicit/Request exchange
 * Refreshing existing leases with Confirm, Renew or Rebind (depending
   on the context).

For now, no Information Request (the case where neither IPv6 address or
prefix are requested) is supported. No support for Reconfigure was added
either, as this is optional (the client manifests clearly to the server
that it does not support Reconfigure). Support for these can be added
later if needed.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>