Returns native attribute name.
Native name of the attribute is a name as it is used on the resource or
as seen by the connector. It is used for diagnostics purposes and may be
used by the connector itself. As the attribute names in XSD have to
comply with XML element name limitations, this may be the only way how to
determine original attribute name.
Returns null if native attribute name is not set or unknown.
The name should be the same as the one used by the resource, if the
resource supports naming of attributes. E.g. in case of LDAP this
annotation should contain "cn", "givenName", etc. If the resource is not
that flexible, the native attribute names may be hardcoded (e.g.
"username", "homeDirectory") or may not be present at all.