Serve students files as binaries
The following endpoints should return a zip file `application/zip` instead of a base64 encoding of the files
- `/api/v1/submission/{id}/files`: Should return a zip file with the files submitted for that specific submission
The contract of that endpoint should follow the javadoc comment here: https://gitlab.epfl.ch/cs107/moodle-autograde/-/blob/795634581ea446beb4fbd5434d8b16a6a2bedf72/autograde-service/src/main/java/ch/epfl/autograde/api/v1/controller/SubmissionController.java#L124