[CommRustModule] add JSI bindings for findUserIdentities RPC

Summary:
This diff exposes FindUserIdentities to JavaScript

Depends on D11998

Test Plan: Tested later in stack by calling rpc on web/native

Reviewers: kamil, bartek

Reviewed By: bartek

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D11999