Feature: Disabling Local Passwords

authd should support disabling the local password, as has been frequently requested by users. Beside disabling the local password completely, it should also be configurable to only disable the local password for some PAM services (e.g. SSH and GDM, but allow the local password for sudo, su, etc.).

## Original description

### Describe the feature

I want to enable authd to all machines in a larger PC room; however, I do not want the accounts to persist after a log out. This makes the need for a local password (kind of) obsolete.
Ideally, authd should be configurable that no local password is needed.

### Describe the ideal solution

By default, local passwords are enabled, but by adding `use_local_passwd` field to the broker config file (or somewhere else), these can be disabled. SSH will not prompt for a local password, and GDM won't either.

### Alternatives and current workarounds

I have not found any workarounds. I suspect I could edit the PAM to make this work, but I do not know enough about PAM to do so.

Start by tracing authd's broker configuration and its PAM integration, using the requested SSH, GDM, sudo, and su behaviors as entry points. Determine how a global setting and per-service settings should be represented and applied. Done means local-password use can be disabled globally or selectively while remaining enabled by default elsewhere.

Written by the indexing model from the issue text.