fix(sssd): replace provider 'files' with 'proxy'

In Ubuntu 26.04, the 'files' provider has been removed, see
https://sssd.io/docs/files-provider-deprecation.html

It can be replaced by the 'proxy' provider, with additional
property 'proxy_lib_name' set to 'files'. This configuration
is compatible with Ubuntu 22.04, 24.04 and 26.04 LTS, and all
intermediate versions.

Solves issue #16

etc/sssd/sssd.conf

@@ -22,7 +22,8 @@ offline_credentials_expiration = 0
 
 
 [domain/ad.francetelecom.fr]
-id_provider = files
+id_provider = proxy
+proxy_lib_name = files
 min_id = 1000
 max_id = 60000
 #