plugin/k8s_external: Add support for PTR requests (#5435)

* Exclude External IP addresses from being added to the existing kubernetes' plugin IP->Service index
* Add support for PTR requests on External IPs of Services to the k8s_external plugin

Signed-off-by: Chris O'Haver <cohaver@infoblox.com>

plugin/k8s_external/transfer_test.go

@@ -59,6 +59,11 @@ func TestTransferAXFR(t *testing.T) {
 			if ans.Header().Rrtype == dns.TypeTXT {
 				continue
 			}
+
+			// Exclude PTR records
+			if ans.Header().Rrtype == dns.TypePTR {
+				continue
+			}
 			expect = append(expect, ans)
 		}
 	}