Implement core_privacy API for gradeimport

Testing can be completed by running the attached testwithmanager.php script (see --- MDL-61571 --- for the original script location) in the Moodle root folder. Make sure you've purged the caches (php admin/cli/purge_caches.php), before running testwithmanager.php. Check that gradeimport plugins are compliant Check that all gradeimport plugins are null providers Check that the null reason is valid You should receive a script output like: Testing the compliant plugins:   .....   == gradeimport_csv (gradeimport_csv\privacy\provider) == Claims not to store any data with reason: 'The import grades from CSV plugin does not store any personal data.' == gradeimport_direct (gradeimport_direct\privacy\provider) == Claims not to store any data with reason: 'The import grades from copied spreadsheed plugin does not store any personal data.' == gradeimport_xml (gradeimport_xml\privacy\provider) == Claims not to store any data with reason: 'The import grades from XML plugin does not store any personal data.'