[identity] move findUserID to unauth proto

Summary:
resolves ENG-6782 and its subtasks

We need to be able to call this RPC from a logged out client to check if an account already exists for a given ETH wallet. In order to call it from a logged out client, it must be in the unauth service.

Test Plan: commtest test passed, flow passed

Reviewers: ashoat, bartek

Reviewed By: ashoat, bartek

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D11052