[lib] rename IdentityRegisterResult -> IdentityAuthResult

Summary:
I'm going to return this type on login, too, so changing register to auth in the type name. open to other suggestions, though.

Depends on D10740

Test Plan: successfully built the app for iOS simulator

Reviewers: ashoat, bartek, inka

Reviewed By: ashoat

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D10741