net: route: Add support for route lifetime

Add support for route lifetime, as defined in RFC 4191. The existing
route adding logic remains the same, if not specified, lifetime is set
to infinite. For routes added with Route Info option from ICMPv6 RA
message, set the expiration timer, according to the route lifetime value
received.

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