Missing sftp support?

Would it be possible to add `sftp` (sshfs) support?

I get this message on the latest stable version, and I'm unable to open any ebooks:

```
Could not open sftp://user@server:22/path/to/book.cbr. Reason: Unable to create KIO worker. Unknown protocol 'sftp'.
```

What's weird, is that other Flatpak apps work fine, I only get this with Okular and using sshfs.

Thanks!