MDL-45216 events: Update to core/event/manager to return observer information.
MDL-45216 events: Unit tests for core/event/manager.

This includes unit tests for the new method get_all_observers
and updates to other methods that have been updated.
MDL-45218 events: New static methods for getting general event information.

New methods are get_static_info() and get_explanation().
MDL-45218 events: Unit tests and fixtures for new static methods in base event class.