plugin/etcd: allow disabling legacy apex fallback (#8468)

Signed-off-by: houyuwushang <liuluoqianqiu@outlook.com>

plugin/backend.go

@@ -37,4 +37,7 @@ type ServiceBackend interface {
 }
 
 // Options are extra options that can be specified for a lookup.
-type Options struct{}
+type Options struct {
+	// NoApexFallback disables the legacy zone-root lookup when apex records are absent.
+	NoApexFallback bool
+}