add new opaque API to get server keypair

Summary: Expose a new API so we can get the keypair used by the server during registration and login. This should be persisted by the server.

Test Plan: successfully called the new api from C++

Reviewers: jimpo, karol-bisztyga, ashoat

Reviewed By: jimpo, ashoat

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

Differential Revision: https://phabricator.ashoat.com/D3156