Allow to use full DN as value for member attribute instead of member: username (#8969)

Allow to use full DN as value for member attribute instead of member: username

I dont use LDAP so this should be tested with both methods.

For using fulldn as user `$config['ldap_auth_userdn'] = true;` must be set in config.php

This comes from https://community.librenms.org/t/feature-request-full-dn-as-group-member-attibute-in-ldap-auth/4805

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includes/defaults.inc.php

@@ -647,6 +647,8 @@ $config['auth_ldap_groupmemberattr']            = 'memberUid';
 $config['auth_ldap_emailattr']                  = 'mail';
 $config['auth_ldap_cache_ttl'] = 300;
 // How long in seconds should ldap* module cache user information in $_SESSION
+$config['auth_ldap_userdn']                     = false;
+// Uses a users full DN as the value of the member attribute in a group (instead of member: username, it’s member: uid=username,ou=groups,dc=domain,dc=com).
 
 // Active Directory Authentication
 $config['auth_ad_user_filter'] = "(objectclass=user)";