[lib] introduce the aux user store ops

Summary:
This diff introduces the aux user store ops spec. This will be used to process aux user store operations.

Depends on D11505

Test Plan: flow check. Functionality tested later in stack.

Reviewers: ginsu, kamil, atul

Reviewed By: atul

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D11506