[native] wallet login method for gRPC client

Summary:
A method that will be exposed in the ffi module to handle wallet login on client side. Included a small helper function in this diff.

Depends on D4927

Test Plan: called the method and got back an access token

Reviewers: tomek, karol, atul

Reviewed By: tomek

Subscribers: ashoat, abosh

Differential Revision: https://phab.comm.dev/D4933