Added course service unit tests

[Devesh21700Kumar]

Description

1). Added course service unit tests 2). Added exec method to facilitate testing and queries 3). added interfaces to support types for objects used in service testing 4). added dummy business logic in service for testing methods without populated instances

• [x] Code
• [x] Testing

How Has This Been Tested? Terminal commands to test code:- 1). npm run lint 2). npm run lint:fix 3). nest start 4). npm run test

Checklist

• [x] My code follows the code style of this project.
• [x] All new and existing tests passed.
• [x] My changes generate no new warnings
• [x] The title of my pull request is a short description of the requested changes.

[Devesh21700Kumar]

@Abhishek-kumar09 Added course service unit tests