Support mapping between types and their corresponding objectClasses.

Currently, the LDAP config requires a baseDn when mapping a type.

In some case, specially for those when the LDAP server is only used for reads, we can get the right type looking at the objectClass attribute values instead.

This is also true when the LDAP server have a complex organization for DNs and map all available DNs is almost impossible and less flexible.

That said, the baseDN should be optional.