Feature: could authd be integrated in sssd as a sub-module?

### Is there an existing request for this feature?

- [x] I have searched the existing issues and found none that matched mine

### Describe the feature

First things first , thank you to the canonical/ubuntu team for making this happen ... i think it's a long awaited PAM module by small to medium teams/companies especially since we don't really want to manage user directories (AD DS, LDAP/Kerberos, RADIUS, etc) and `authd` really brings some sanity for having user auth on our devices using our org's said IdP that's also our email service provider.
So a big, Thank You!

Now this is mostly a question , not really any kind of request or problem ... but pure curiosity.
Since SSSD has so many sub-modules (ldap, ad, kerberos, etc) , couldn't `authd` be also be implemented as an sssd submodule ?
Not sure if `sssd` is an independed project or if it's redhat managed , i guess that might be one issue if it's maintained by redhat. In any case , i used sssd on ubuntu some years ago and it was quite easy to set-up and it provided pretty much all the required configuration need to make it suit any need (it was used with LDAP/OpenLDAP)

Pretty much any configuration directive that you could think of , it was already there (either in the main app or the sub-module) + credential caching.

### Describe the ideal solution

_No response_

### Alternatives and current workarounds

_No response_

Start by reviewing authd's integration boundary and SSSD's submodule interfaces; the issue names no files, tests, or entry points. Done would require a decided feasibility and design path for integrating authd as an SSSD submodule, since no concrete acceptance criteria are given.

Written by the indexing model from the issue text.