Add a cache definition for grade categories

1. It SHOULD pass all behat tests
2. It SHOULD pass all phpunit tests

This is the real test:

1. We are adding a cache for grade_categories (not the grade categories grade_items data). The grade categories data gets updated through the form, but for example, a gradebook recalculation shouldn't update grade categories data. In general we need to check that cache invalidation is working as expected, you can feel free to test what you want, but the basic test I think is required is to:
   1. Visit 'Site administration' > 'Grades' > 'Grade category settings' and enable all aggregation types.
   2. Go to a course grader report with at least 2 activities and one grade category using any 'Aggregation' method except for 'Natural'.
   3. Open the gradebook setup in a new tab and go to the grade category edit form, save without changing any value and return to the grader report, grades should be the same in the other tab.
   4. Now edit the category again and change the 'Maximum grade' value in the grade category form (remember the previous values, you will need to restore them) and check that the grader report grades change according to it, keep that tab opened with that data and open a new one going to gradebook setup again
   5. Restore the grade category previous values and go to the grader report to compare the grades with the initial tab, they should be the same
   6. Thank for testing this