[reports-service] Endpoint for viewing single report

Summary:
Endpoint to view a single report, given its ID.

Depends on D8970

Test Plan: Used Postman to test the endpoint for a few various reports.

Reviewers: michal, patryk, varun, jon

Reviewed By: michal

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D8971