luci-proto-external: migrate from support to cni-protocol to external-protocol

luci-proto-external: migrate from support to cni-protocol to external-protocol

As previously introduced cni protocol support (for netifd) has evolved more
to a multipurpose protocol useful for cni/netavark/some vpn/etc connections
I have decided to rename it from cni-protocol to external-protocol.

That's why also it's luci companion, luci-proto-cni, should be renamed
to luci-proto-external and migrated to support external-protocol.

This patch also adds support for new option in external-protocol, delay.

This closes promise made in #22702 on openwrt packages repository.

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