[lib] modify set aux user fids and replace old behavior with add aux user fids action type

Summary:
Following feedback on D12011, decided to refactor the aux user store actions.

In D12011, we want a single action type that both clears prior fids and adds payload fids.

As setAuxUserStoreFIDs has yet to be used anywhere in our codebase, I have the action type first clear fids and add payload fids.

The previous functionality of just adding fids becomes addAuxUserFIDs.

Depends on D12095

Test Plan: added aux user reducer tests

Reviewers: kamil, ginsu, ashoat

Reviewed By: ashoat

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D12105