[client-backup] implement API call to upload backup

Summary:
Created multipart form data and uploads it with authentication.

Depends on D8992

Test Plan: Call this with mock data and check if works, call it with wrong data/auth to make sure it will be rejected.

Reviewers: michal, marcin, bartek

Reviewed By: marcin, bartek

Subscribers: patryk, ashoat, tomek

Differential Revision: https://phab.comm.dev/D8993