Design the plugin configuration

[simonw]

Inspired by https://github.com/simonw/datasette-auth-tokens/blob/d6f45da578419177c260628db5f8352a839e45f2/README.md

[simonw]

The first release will just handle hard-coded accounts - its configuration can look like this:

{
    "plugins": {
        "datasette-auth-passwords": {
            "accounts": {
                "my_username": {
                    "password_hash": {
                        "$env": "ROOT_PASSWORD_HASH"
                    },
                    "actor": {
                        "id": "root"
                    }
                }
            }
        }
    }
}