feat: add dns.useStagingACME option

Used for testing environments, so we don't stumble upon ACME rate limits.

letsencrypt/acme.nix

@@ -8,7 +8,10 @@ in
   };
   security.acme = {
     acceptTerms = true;
-    email = "${cfg.username}@${cfg.domain}";
+    defaults = {
+      email = "${cfg.username}@${cfg.domain}";
+      server = if cfg.dns.useStagingACME then "https://acme-staging-v02.api.letsencrypt.org/directory" else "https://acme-v02.api.letsencrypt.org/directory";
+    };
     certs = lib.mkForce {
       "${cfg.domain}" = {
         domain = "*.${cfg.domain}";