New "universal" authenticator for types "dynamic" and "function"

[oberstet]

currently, when one wants to configure dynamic or function based authenticators, this needs to be declared for each authmethod that is desired.

writing authenticators that work for any authmethod would be more convenient.

we can do this with a new universal authenticator, instead of

"auth": {
    "anonymous": {
        "type": "function",
        "create": "authenticator.create_authenticator",
        "expose_controller": true,
        "config": {
            "dbname": "foo"
        }
    },
    "cryptosign": {
        "type": "function",
        "create": "authenticator.create_authenticator",
        "expose_controller": true,
        "config": {
            "dbname": "foo"
        }
    },
    "scram": {
        "type": "function",
        "create": "authenticator.create_authenticator",
        "expose_controller": true,
        "config": {
            "dbname": "foo"
        }
    }
}

we could have

"auth": {
    "universal": {
        "type": "function",
        "create": "authenticator.create_authenticator",
        "expose_controller": true,
        "config": {
            "dbname": "foo"
        }
    }
}

[oberstet]

see also #1807