[lib] Make FarcasterDataHandler go through useFindUserIdentities

Summary:
There are currently two components that call findUserIdentities: UserInfosHandler and FarcasterDataHandler.

The former goes through useFindUserIdentities, but the latter accesses findUserIdentities directly via the IdentityClientContext context.

This diff makes both of them go through useFindUserIdentities, as preparation for adding a caching layer to that hook.

This is the first step to addressing ENG-9002.

Test Plan: This is a simple refactor. I made sure that IdentityClientContext should always be set where FarcasterDataHandler is rendered on both native and web.

Reviewers: varun, will

Reviewed By: varun

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D13117