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.
this makes it easier for one to manually administer freshrss.
for example, i can import OPML from the CLI like:
```
$ nix build .#freshrss
$ freshrss FRESHRSS_DATA_PATH=/var/lib/freshrss ./result/cli/import-for-user.php --user admin --file my-opml.opml
```
whereas previously i would have needed to include
`environment.systemPackages = [ php ];` in my system for that to work.
