external-protocol: rename and update cni-protocol

external-protocol: rename and update cni-protocol

cni-protocol can be used for both cni and netavark
and also for many other things, such as vpn's that
lack customized protocol supports for openwrt as a
general externally managed protocol, so it was due
to rename it.

I also added one extra option, search domain, which
is optional and updated scripts retrieving ip address
and routing information.

Signed-off-by: Oskari Rauta <oskari.rauta@gmail.com>