Implement providers for core_register

These changes are covered by the unit tests for the manager. Also, you can testing 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 cache (php admin/cli/purge_caches.php), before running testwithmanager.php. You should receive a script output like: Testing the compliant plugins: ... == core_register (core_register\privacy\provider) == Claims not to store any data with reason: 'The Register plugin does not store any personal data.' ...