Implement core_privacy for core_calendar

Testing steps: Moodle 3.5/3.4: Run PHP unit test calendar/tests/privacy_test.php Log in as Admin and create Calendar Events at Site, User, Category, Course context levels. (NOTE: Moodle 33 does not support Calendar Events / Subscriptions at the Category context level). As Admin create a Calendar Subscription at Site, User, Category, and Course context levels. As Admin create a Course and add a new Assignment with duedate, and gradingduedate values. Repeat creating Calendar Events and Subscriptions as a Teacher and Student. Run testwithmanager.php script to test core_calendar privacy compliance and data export (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. You should receive a script output like: Testing the compliant plugins: .....   == core_calendar (core_calendar\privacy\provider) ==     Found 1 items of metadata     Successfully ran a test export A single Calendar Event at a context level, time, and eventtype should be exported with JSON file path/name "Calendar/Events/{timestart}/{eventtype}-event.json", Multiple Calendar Events at the same context level, time, and eventtype should be exported with JSON file name/path "Calendar/Events/{timestart}/{eventtype}-event- {no} .json". A single Calendar Subscriptions at a context level, and eventtype should be exported with JSON file path/name "Calendar/Subscriptions/{eventtype}-subscription.json". Multiple Calendar Subscriptions at the same context level, and eventtype should be exported with JSON file name/path "Calendar/Subscriptions/{timestart}/{eventtype}-subscription-{no}.json". Moodle 3.3: Repeat testing steps above without setting Calendar Events / Subscriptions at the Category context levels.