[lib] Modify siweAuth(...) return type from LogInResponse to LogInResult

Summary:
We construct LogInResult using the request payload (SIWEAuthServerCall) and the respond (LogInResponse). Code was effectively cut/paste from lib/actions/user-actions:logIn.

Depends on D6077

Test Plan: Will be tested implicitly by subsequent diffs.

Reviewers: ashoat, tomek

Reviewed By: ashoat, tomek

Differential Revision: https://phab.comm.dev/D6085