[client-backup] add initial backup types

Summary:
Types to handle information about backup.

UserKeys - was discussed on crypto sync.
UserData - selecting store which not updates very often because in initial version we perform full backup, so this should avoid slowing down app too much.
BackupAuth - D8911

Depends on D8991

Test Plan: N/A

Reviewers: michal, marcin, bartek

Reviewed By: marcin

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D8992