net: config: Add DHCPv6 support

Add DHCPv6 support to the net config library. In case DHCPv6 is enabled,
net config will attempt to acquire IPv6 address and/or prefix when used.
The user can select with Kconfig whether to request address or prefix
(or both).

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