[native_rust_library] refactor helper structs to avoid duplication

Summary: Rather than introduce more optional fields to the existing structs for reserved password and wallet registration, I decided to refactor the structs entirely.

Test Plan: registered and logged in a wallet user and a password user on native. verified that DDB has all the correct keys and other info

Reviewers: will, bartek

Reviewed By: bartek

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D11861