Add DnsWriter for Cloud DNS

The plumbing to support different DnsWriter implementations per TLD
using DI will come in a later CL.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=129375856

apiserving/discoverydata/cloud/dns/BUILD

@@ -0,0 +1,9 @@
+package(default_visibility = ["//visibility:public"])
+
+java_library(
+    name = "cloud_dns_v2beta1_versioned",
+    exports = ["@google_api_services_dns//jar"],
+    runtime_deps = [
+        "@google_api_client//jar",
+    ],
+)