Added s3 as a data source for attachments that are grader_tests

Summary: Now tests can be stored in S3 instead of the main server, greatly reducing storage costs

Test Plan: Install a vm and test upload,download,rename(BLOCKED) and delete

Reviewers: vduta, bogdan2412

Reviewed By: bogdan2412

Subscribers: echipa-infoarena

Differential Revision: https://hackers.infoarena.ro/D30