[native] introduce some structs for login and registration

Summary:
These structs serve two purposes:

1. Reduce the number of function params
2. Make helper functions more extensible by exposing these structs as enum variants

Test Plan: cargo build (dead code currently)

Reviewers: tomek, karol, atul

Reviewed By: tomek

Subscribers: ashoat, abosh

Differential Revision: https://phab.comm.dev/D4876