Add a base search parameter for ldap group sync and a filter

[goudal-emmk]

Hello,
The plugin is nice, but we should be able to add a base search when looking for groups.
I explain : the ldap directory can be used to store groups for different clients and different purposes. Not all groups are dedicated to moodle. So we should be able to select which groups we want to use in moodle. The objectClass is not enough selective.
Adding a filter and a base search in the plugin configuration would do the job, so we can select the groups we want to use.
Thanks in advance...