Implement core_privacy API for core_rss

These changes are covered by the unit tests in rss/tests/privacy_test.php 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 executed these steps and purged the cache  (php admin/cli/purge_caches.php), before running testwithmanager.php: Login as admin. Access to "Site administration | Advanced features" and " Enable RSS feeds ". Access to "Site administration | Plugins | Activity modules | Forum" and select "Yes" for the " Enable RSS feeds " setting. Create a course with a forum, and select "Posts" in "RSS feed for this activity" and 5 in "Number of RSS recent articles".   You should receive a script output like: Testing the compliant plugins: ... == core_rss (core_rss\privacy\provider) == Found 1 items of metadata Successfully ran a test export ...