[31/10/2022] Backend API [feat] - Enroll course

[RealRichi3]

Sub issue from #5

Description

Each user should be able to enroll for a course, and the user should be able to get the list of courses enrolled.
The course schema should also have a way to get the list of users that enrolled.
It'll be like a one to many relationship from the courses to the users
Users should also be able to cancel their enrollment

What needs to be done?

• Add enrolled course field to user schema
• Add enrolled users to course schema
• Add enroll course feature
• Add GET enrolled users
• Add GET enrolled course by user
• Add CANCEL course enrollment