net: dns: Allow creating context with specific svc and port

Allow user to specify socket service struct and port number so
that DNS servers specified in Kconfig etc are not used. This way
we can send a DNS query to arbitrary address without it affecting
the system configuration.

This is used in mDNS probing so that that mDNS responder can send
probe message and not change / configure system global DNS config.

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