Add new plugin: external - resolve k8s ingress and LB address with external names (#2379)

* 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>

plugin/k8s_external/README.md

@@ -0,0 +1,78 @@
+# k8s_external
+
+## Name
+
+*k8s_external* - resolve load balancer and external IPs from outside kubernetes clusters.
+
+## Description
+
+This plugin allows an additional zone to resolve the external IP address(es) of a Kubernetes
+service. This plugin is only useful if the *kubernetes* plugin is also loaded.
+
+The plugin uses an external zone to resolve in-cluster IP addresses. It only handles queries for A,
+AAAA and SRV records, all others result in NODATA responses. To make it a proper DNS zone it handles
+SOA and NS queries for the apex of the zone.
+
+By default the apex of the zone will look like (assuming the zone used is `example.org`):
+
+~~~ dns
+example.org.	5 IN	SOA ns1.dns.example.org. hostmaster.example.org. (
+				12345      ; serial
+				14400      ; refresh (4 hours)
+				3600       ; retry (1 hour)
+				604800     ; expire (1 week)
+				5          ; minimum (4 hours)
+				)
+example.org		5 IN	NS ns1.dns.example.org.
+
+ns1.dns.example.org.  5 IN  A    ....
+ns1.dns.example.org.  5 IN  AAAA ....
+~~~
+
+Note we use the `dns` subdomain to place the records the DNS needs (see the `apex` directive). Also
+note the SOA's serial number is static. The IP addresses of the nameserver records are those of the
+CoreDNS service.
+
+The *k8s_external* plugin handles the subdomain `dns` and the apex of the zone by itself, all other
+queries are resolved to addresses in the cluster.
+
+## Syntax
+
+~~~
+k8s_external [ZONE...]
+~~~
+
+* **ZONES** zones *k8s_external* should be authoritative for.
+
+If you want to change the apex domain or use a different TTL for the return records you can use
+this extended syntax.
+
+~~~
+k8s_external [ZONE...] {
+    apex APEX
+    ttl TTL
+}
+~~~
+
+* **APEX** is the name (DNS label) to use the apex records, defaults to `dns`.
+* `ttl` allows you to set a custom **TTL** for responses. The default is 5 (seconds).
+
+# Examples
+
+Enable names under `example.org` to be resolved to in cluster DNS addresses.
+
+~~~
+. {
+   kubernetes cluster.local
+   k8s_external example.org
+}
+~~~
+
+# Also See
+
+For some background see [resolve external IP address](https://github.com/kubernetes/dns/issues/242).
+And [A records for services with Load Balancer IP](https://github.com/coredns/coredns/issues/1851).
+
+# Bugs
+
+PTR queries for the reverse zone is not supported.