Allow the usage of the plugin for other auth methods than auth_ldap_syncplus

[abias]

There may be Moodle setups where this plugin should be used for provisioning and deprovisioning Moodle users, but where these users are not authenticated with ldap_syncplus.

An example would be the provisioning and deprovisioning of Shibboleth users. Shibboleth does not necessarily have a way to provision and deprovision users, but a Shibboleth IDP might be able to additionally expose its user base via LDAP which would then allow this plugin to do the job.

To realize this goal, an additional setting would be added which allows the admin to set the authtype of the users who are created, updated or deleted by this plugin.