nixos/nginx: make redirect status code configurable

Add an option to configure which code globalRedirect and forceSSL use.
It previously was always 301 with no easy way to override.

nixos/tests/nginx-redirectcode.nix

@@ -0,0 +1,25 @@
+import ./make-test-python.nix ({ pkgs, lib, ... }: {
+  name = "nginx-redirectcode";
+  meta.maintainers = with lib.maintainers; [ misterio77 ];
+
+  nodes = {
+    webserver = { pkgs, lib, ... }: {
+      services.nginx = {
+        enable = true;
+        virtualHosts.localhost = {
+          globalRedirect = "example.com/foo";
+          # With 308 (and 307), the method and body are to be kept when following it
+          redirectCode = 308;
+        };
+      };
+    };
+  };
+
+  testScript = ''
+    webserver.wait_for_unit("nginx")
+    webserver.wait_for_open_port(80)
+
+    # Check the status code
+    webserver.succeed("curl -si http://localhost | grep '^HTTP/[0-9.]\+ 308 Permanent Redirect'")
+  '';
+})