Expose assignments and grades through REST API

[kirkroyster]

Is your feature request related to a problem? Please describe.
The need / gaps are two-fold: 1) absence of "assignment" object in several routes: /sections/id/content/id; /lessons/id; 2) absence of grade data for: student-course; student-assignment; student-quiz. Assignments is needed in order to get all grades (course, quiz, assignment) for a student. This functionality would round out the existing API scope.

Describe the solution you'd like
Bring the prototype snippet that we have successfully implemented, into the core API. For reference, refer to the following Slack link:
https://app.slack.com/client/T0H18E2VB/D04Q51QTKDX/thread/C0H1CE4SW-1676315733.221279
(refer to Rocco Aliberti of LLMS).

Describe alternatives you've considered
This is the only alternative that enables us to import full student/course/grade data into our transactional system. We need our transactional system as we also have a much larger number of "on-site" students and various custom requirements such as detailed report cards that are generated off of the LLMS sections/content text, etc.