[services] Backup - add relevant opaque apis for server

Summary: this diff exposes the server apis from the opaque-ke library for account registration and login. wanted to get this out before it blocks the backup service.

Test Plan: unit tested, checked that the C++ headers were generated as expected

Reviewers: jimpo, karol-bisztyga

Reviewed By: jimpo

Subscribers: benschac, ashoat, palys-swm, Adrian, atul

Differential Revision: https://phabricator.ashoat.com/D3173