Add Google Cloud DNS plugin (#3011)

Signed-off-by: Palash Nigam <npalash25@gmail.com>

Closes: #2822

plugin/clouddns/README.md

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+# clouddns
+
+## Name
+
+*clouddns* - enables serving zone data from GCP clouddns.
+
+## Description
+
+The clouddns plugin is useful for serving zones from resource record
+sets in GCP clouddns. This plugin supports all [Google Cloud DNS records](https://cloud.google.com/dns/docs/overview#supported_dns_record_types).
+The clouddns plugin can be used when coredns is deployed on GCP or elsewhere.
+
+## Syntax
+
+~~~ txt
+clouddns [ZONE:PROJECT_NAME:HOSTED_ZONE_NAME...] {
+    credentials [FILENAME]
+    fallthrough [ZONES...]
+}
+~~~
+
+*   **ZONE** the name of the domain to be accessed. When there are multiple zones with overlapping
+    domains (private vs. public hosted zone), CoreDNS does the lookup in the given order here.
+    Therefore, for a non-existing resource record, SOA response will be from the rightmost zone.
+
+*   **HOSTED_ZONE_NAME** the name of the hosted zone that contains the resource record sets to be
+    accessed.
+
+*   `credentials` is used for reading the credential file.
+
+*   **FILENAME** GCP credentials file path.
+
+*   `fallthrough` If zone matches and no record can be generated, pass request to the next plugin.
+    If **[ZONES...]** is omitted, then fallthrough happens for all zones for which the plugin is
+    authoritative. If specific zones are listed (for example `in-addr.arpa` and `ip6.arpa`), then
+    only queries for those zones will be subject to fallthrough.
+
+*   **ZONES** zones it should be authoritative for. If empty, the zones from the configuration block
+
+## Examples
+
+Enable clouddns with implicit GCP credentials and resolve CNAMEs via 10.0.0.1:
+
+~~~ txt
+. {
+	clouddns example.org.:gcp-example-project:example-zone
+    forward . 10.0.0.1
+}
+~~~
+
+Enable clouddns with fallthrough:
+
+~~~ txt
+. {
+    clouddns example.org.:gcp-example-project:example-zone clouddns example.com.:gcp-example-project:example-zone-2 {
+      fallthrough example.gov.
+    }
+}
+~~~
+
+Enable clouddns with multiple hosted zones with the same domain:
+
+~~~ txt
+. {
+    clouddns example.org.:gcp-example-project:example-zone example.com.:gcp-example-project:other-example-zone
+}
+~~~