Support of hashed passwords when installing tljh

### Proposed change

Hi guys, it will be great to add support of hashed passwords in firstuseauthenticator. It will greatly helps in tljh installation automation process when you need to create jupyterhub with bunch of predefined users and passwords and also this feature will not compromise passwords when you store them in install process.

So for example for password _test_ hashed with sha512sum tljh install will look

```
curl -L https://tljh.jupyter.org/bootstrap.py | python3 - \
--admin admin:ee26b0dd4af7e749aa1a8ee3c10ae9923f618980772e473f8819a5d4940e0db27ac185f8a0e1d5f84f88bc887fd67b143732c304cc5fa9ad8e6f57f50028a8ff
```

Start by tracing how firstuseauthenticator handles password creation and how the tljh install command parses the --admin value. Confirm the expected hash format and authentication behavior, then add coverage showing that hashed passwords are accepted without requiring plaintext passwords.

Written by the indexing model from the issue text.