add Local variant

### Proposed change

define a `LocalFirstUseAuthenticator` that creates system users. Most authenticators (OAuthenticator and friends) support this, and it generally consists of:

```python
class LocalFirstUseAuthenticator(FirstUseAuthenticator, LocalAuthenticator):
pass
```

### Alternative options

reimplement local users another way

### Who would use this feature?

users of FirstUseAuthenticator with the default Spawner or systemd or other system-user-requiring spawner. See https://github.com/jupyterhub/jupyterhub/issues/2948

### (Optional): Suggest a solution