Base functionality for ENG-11192.
Updated the hook to also search for Farcaster DCs users if the flag is set. This will be used, for example, to add non-Comm members to Farcaster threads.

Depends on D15278

• Tested that old behavior didn't change (searchFarcaster = false).
• Switched the flag and tested that Farcaster users show up in search results (used the add friend interface in user profile)