nixos/freshrss: authType option

This patch adds an `authType` option to enable configuring FreshRSS's
`auth_type` parameter.
Upstream documentation for this feature is located here:
https://freshrss.github.io/FreshRSS/en/admins/09_AccessControl.html

An accompanying NixOS test is provided to confirm this feature works
as expected.

nixos/tests/freshrss-http-auth.nix

@@ -0,0 +1,20 @@
+import ./make-test-python.nix ({ lib, pkgs, ... }: {
+  name = "freshrss";
+  meta.maintainers = with lib.maintainers; [ mattchrist ];
+
+  nodes.machine = { pkgs, ... }: {
+    services.freshrss = {
+      enable = true;
+      baseUrl = "http://localhost";
+      dataDir = "/srv/freshrss";
+      authType = "http_auth";
+    };
+  };
+
+  testScript = ''
+    machine.wait_for_unit("multi-user.target")
+    machine.wait_for_open_port(80)
+    response = machine.succeed("curl -vvv -s -H 'Host: freshrss' -H 'Remote-User: testuser' http://127.0.0.1:80/i/")
+    assert 'Account: testuser' in response, "http_auth method didn't work."
+  '';
+})