Expose assignments and grades through REST API

**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.