[backup] Introduce download endpoints

Summary:
ENG-4501

Endpoints for downloading backup blobs: user_keys and user_data.

Test Plan:
Run:

GET http://127.0.0.1:50052/backups/id7/user_keys
Authorization: Bearer eyJ1c2VySUQiOiAiMSIsICJhY2Nlc3NUb2tlbiI6ICIyIiwgImRldmljZUlEIjogIjMifQ==

GET http://127.0.0.1:50052/backups/id7/user_data
Authorization: Bearer eyJ1c2VySUQiOiAiMSIsICJhY2Nlc3NUb2tlbiI6ICIyIiwgImRldmljZUlEIjogIjMifQ==

Test if the returned data is correct. Run with non-existant backup id, check if 404 is returned.

Reviewers: bartek, patryk, kamil, jon

Reviewed By: bartek

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D8964