[API] Add Associated Course ID for Assignment Associations of a Rubric

Problem statement:

Using the Canvas API, it is possible to determine which accounts, courses, and assignments a rubric is associated with. This is useful when auditing rubric usage. However, when trying to track down an assignment association, this can be tedious when it has been associated with multiple courses. Since you need to know the course ID to get the assignment details, it requires going through each course until you find the one that has the specified assignment ID.

Proposed solution:

To make it easier to locate associated assignments, it would be useful to include the course ID as an additional property in the RubricAssociation object when it is an assignment association. Another possible solution would be to provide an Assignment endpoint that allows getting assignment details using only the assignment ID. i.e. "/api/v1/assignments/12345"

User role(s):

admin,instructor,ta,designer