[4/n] Native Typeahead - Add user search and thread members to props

Summary:
Preparing for native typeahead.
We will need threadMembers and searchIndex to find suggested users based on what current user typed in input bar.

Test Plan:
Checked if app builds.
Will use those props in later diffs.

Reviewers: inka, ginsu, kamil, michal, tomek

Reviewed By: inka, tomek

Subscribers: ashoat, tomek, atul, michal, kamil, ginsu, inka

Differential Revision: https://phab.comm.dev/D6300