[identity][comm-lib] move is_valid_ethereum_address to comm-lib

Summary:
ENG-6145.

We'll need to use this on backup, to be able to call FindUserID RPC based on public user identifier - we need to somehow know if this is a Password or Wallet user

Test Plan: Run unit tests in Identity and comm-lib

Reviewers: varun, bartek

Reviewed By: varun, bartek

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D13803