[lib] Fetch user avatars for new user ids

Summary:
issue: ENG-7823
After calling update relationships endpoint, the keyserver will generate updates and send them to the client. This way we will obtain the avatar.
For now, this action will override the username for this id if already present in the store. This will be changed in ENG-8365.

Test Plan: Dispatched processNewUserIDsActionType with user id that the current user didn't have in the store. Checked that user infos are fetched from the keyserver and saved in the store.

Reviewers: tomek, kamil

Reviewed By: tomek

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D12319