net: utils: Add helper to parse ip address string with a mask len

Adding a helper to parse one or more IPv4/6 strings with optional
netmask or prefix length.

Example of the string:

   2001:db8::1/64,192.0.2.1,2001:db8::2,192.0.2.2/24

Signed-off-by: Jukka Rissanen <jukka.rissanen@nordicsemi.no>