[lib] Include searchUsers results in getPotentialMemberItems

Summary: We will want to include results from user search endpoint in web and native chat composers. This diff adds an option to getPotentialMemberItems to include additional users from the endpoint.

Test Plan: Tested if the user are returned correctly (that are not in user store) are returned correctly

Reviewers: kamil, inka

Reviewed By: kamil

Subscribers: ashoat, tomek, atul

Differential Revision: https://phab.comm.dev/D8009