feat(medcat-den): Make credentials optional for remote dens

[alhendrickson]

Current Behavior

• I have a medcattery instance with no authentication, this is the default when I start it up.
• I can't use medcat-den without passing an unused credentials block

EG I have to do this in the json:

        "medcattery_prod": {
            "type": "medcattery",
            "host": "https://medcattery.example.com",
            "credentials": {
                "unused: "unused"
            }
        }

New behavior

I should now be able to skip that credentials block altogether

        "medcattery_prod": {
            "type": "medcattery",
            "host": "https://medcattery.example.com"
}

(Note I also need to change medcat-den-medcattery implementation)

[mart-r]

Yeah, why not. I've had the same issue before... So this would certain simplify some stuff