Feature: Support overriding the home directory via a template

SSSD supports the `override_homedir` option in [sssd.conf](https://manpages.ubuntu.com/manpages/noble/man5/sssd.conf.5.html) to provide a template which is used to assemble the home directory that's used for a new user.

These are some of the substitutions it supports:

```
%u
login name

%U
UID number

%d
domain name

%f
fully qualified user name (user@domain)
```

So setting the option to `/home/%u` creates the home directory as `/home/`.

Users requested that authd should support something similar in https://github.com/ubuntu/authd/discussions/884.

No authd file, test, or entry point is named in the issue. Start by reviewing authd's existing home-directory handling and the referenced SSSD override_homedir behavior, then identify the supported substitutions and verify that a configured template produces the expected path for a new user.

Written by the indexing model from the issue text.