[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