Add exercise endpoints GET all and DELETE one exercise

Description

Adds the endpoint for an instructor to GET all exercises he authored.
Adds the endpoint for an instructor to DELETE one of his exercises, by providing its ID.

Part of #53.

Adds GetExercisesResponseDto
Adds ExerciseService
Adds new functionality to the above described endpoints, but on API, Core, and Infrastructure level.

The PR does not close #53 as it does not provide a GET endpoint for a single exercise. This has further dependencies described in the issue.
The solution for deletion of an exercise was at first inspired by the deletion of a session, which was existing functionality, however, and error was detected:
- When playing with requests it was found that providing a different authorId, than the one associated to the exercise, still resulted in the exercise being deleted in the database. A Bug will be created on this issue (#70).