[native] change params and types to satisfy cxx requirements

Summary:
Changed auth_type to an i32 to make it easier to pass across the FFI boundary. Also changed a return type to a String for the same reason.

Depends on D4933

Test Plan: cargo build

Reviewers: tomek, karol, atul

Reviewed By: tomek

Subscribers: ashoat, abosh

Differential Revision: https://phab.comm.dev/D4934