Implement core_privacy for core_calendar

Testing steps:

Moodle 3.5/3.4:

1. Run PHP unit test calendar/tests/privacy_test.php
2. 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).
3. As Admin create a Calendar Subscription at Site, User, Category, and Course context levels.
4. As Admin create a Course and add a new Assignment with duedate, and gradingduedate values.
5. Repeat creating Calendar Events and Subscriptions as a Teacher and Student.
6. 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

7. 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",
8. 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".

9. A single Calendar Subscriptions at a context level, and eventtype should be exported with JSON file path/name "Calendar/Subscriptions/{eventtype}-subscription.json".
10. 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.