net: dns: Save info about source when configuring DNS servers

Remember which DNS server was added by a source like DHCPv4 or v6
message. This will allow system to remove DNS servers that were added by
that source. Then when stopping for example DHCP, we can remove those
specific DNS servers and not leaving DNS servers hanging in the system.

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