* Add new plugin: external

This plugin works in conjunction with the kubernetes plugin and exports
ingress and LB addresses as DNS records. It bypasses backend.go and
backend_lookup.go flow because it is not needed.

README, tests are implemented. The tests only exercise the unit tests,
this has not been tested in any ci.
Signed-off-by: Miek Gieben <miek@miek.nl>

* Rename to k8s_external
Signed-off-by: Miek Gieben <miek@miek.nl>

* go gen
Signed-off-by: Miek Gieben <miek@miek.nl>