[native][lib] Add RestoreUser RPCs to Identity client

Summary:
Exposed RestoreUser RPCs in Identity Client Context. It's done only on native because it's primary-device-only functionality

Depends on 13231

Test Plan:
Made sure Identity RPCs are getting called.
Also got working test plan from D13226 - replaced login RPCs with this one, but this is yet hacky, without other parts of backup restore protocol.

Reviewers: varun, kamil

Reviewed By: kamil

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D13232