create new aux user store action for clearing fids

Summary:
This new action types clears any aux user store users of fids by setting them all to null.

Depends on D11671

Test Plan: Dispatched this action successfully and confirmed all aux user store entries contained null fid values (they were strings before).

Reviewers: ashoat, atul, ginsu

Reviewed By: atul

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D11672