net: dhcpv4: update DNS address assignment to be optional

The DHCPv4 options always assigns the DNS address if the router
provides one through the DHCP callback. There are times a developer
may not always want to assign the server from the router, but only
manually assign them. Adding a Kconfig option for assigning the DNS
address and defaulting to true to not interfere with previously
expected functionality.

Signed-off-by: Kris Wolff <kwolff@wavelynx.com>