[native_rust_library] Extend findUserIdentities response

Summary:
Modified native_rust_library code to process the new response field. JS changes are made in the next diff.

Depends on D12924

Test Plan: Played with useFindUserIdentities() and made sure it still works as expected.

Reviewers: varun

Reviewed By: varun

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D12925