net: ipv6: mld: add option to include multicast routes in MLDv2 reports

This commit introduces a new option CONFIG_NET_MCAST_ROUTE_MLD_REPORTS
which appends multiacst routes to MLDv2 reports. These records are
intended to inform reports' recipients about potential listeners on
the other interfaces.

Signed-off-by: Konrad Derda <konrad.derda@nordicsemi.no>