[native/web] make findUserIdentities RPC available to native and web

Summary:
Implements FindUserIdentities RPC for web and native

Depends on D11999

Test Plan:
On both web/native successfully called the FindUserIdentities rpc on

1. ethereum user
2. username user
3. user with farcaster id
4. non-existent user (resulting in no entry in the identities result)

Reviewers: kamil, bartek

Reviewed By: bartek

Subscribers: inka, ashoat, tomek

Differential Revision: https://phab.comm.dev/D12000