[identity] Make sure username users can't use ETH addresses as usernames on the identity service

Summary:
regex check to make sure users don't choose a valid ETH address as their username

we already use the regex crate in native_rust_library so it isn't new

Test Plan: unit tests

Reviewers: bartek, kamil, jon

Reviewed By: bartek

Subscribers: ashoat, tomek, wyilio

Differential Revision: https://phab.comm.dev/D9440