[lib] Fetch missing usernames

Summary:
issue: ENG-7743
We can now fetch the usernames using the identity RPC

Test Plan: tested that when new user infos without usernames are added to the store, the effect is run and the usernames are fetched from identity, and inserted into user infos in the user store.

Reviewers: tomek, kamil

Reviewed By: tomek

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D12219