feat: add keys systemd serviceName option

[amerocu]

Adds a new option in the keys attribute set to override the key service name. The default service name remains the name of the key suffixed with "-key" BUT the semantics changes a bit.

With this key declaration:

{
  deployment.keys.my-secret {
    name = "my-pretty-secret";
    ....
  } 
}

Before the service would have called my-secret-key while then it will be called my-pretty-secret-key. I think it's better to use the actual value of key name instead of the default value for the option key name.

This change also allows referring to the service name with the option config.deployment.keys.my-secret.serviceName in other services.

Fixes #141