[reports-service] Endpoint for listing reports

Summary:
A simple endpoint for browsing reports. Counterpart of this keyserver function.
Note that the existing keyserver endpoint is not used anywhere and its functionality is limited and so is the reports-service endpoint.

In the future, we can add some sorting and filtering options.

Depends on D8972

Test Plan:

1. Uploaded 20+ reports to the service.
2. Opened the endpoint in postman. A JSON array with basic report objects was displayed. The "nextPage" contained some value
3. Opened the endpoint again with ?cursor={nextPage value} query string and got the next page of results.
4. On the last page, the "nextPage" value was null.

Reviewers: michal, patryk, varun, jon

Reviewed By: michal

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D8973