Add option to crash `autorestic` when key is missing instead of generating a new key

[dotboris]

Relates to #202

This PR adds the requireKey field to the backend config.

Normally, when autorestic check finds a backend without a key, it will generate a new key and update the config to add this key. This option prevents this behavior. When requireKey: true is set on a backend and no key is found, autorestic will crash instead of generating a key and updating the config.

This should help accidental problems from misconfiguration when managing auotrestic through configuration management tools.