add support for ExternalName service type in dynamic mode

rootfs/etc/nginx/lua/test/dns_helper.lua

@@ -0,0 +1,27 @@
+local _M = {}
+
+local configuration = require("configuration")
+local resolver = require("resty.dns.resolver")
+local old_resolver_new = resolver.new
+
+local function reset(nameservers)
+  configuration.nameservers = nameservers or { "1.1.1.1" }
+end
+
+function _M.mock_new(func, nameservers)
+  reset(nameservers)
+  resolver.new = func
+end
+
+function _M.mock_dns_query(response, err)
+  reset()
+  resolver.new = function(self, options)
+    local r = old_resolver_new(self, options)
+    r.query = function(self, name, options, tries)
+      return response, err
+    end
+    return r
+  end
+end
+
+return _M