net: dns: Allow creating context with specific interface

Allow user to specify the network interface where the query should
be sent, but only if the user has supplied the DNS servers in sockaddr
list. If user has specified a DNS servers in a string list, then the
possible network interface is taken from the string.

The reason for this is that it is not possible to add network interface
to the sockaddr list so an extra parameter is needed.

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