[lib] introduce getEthAddressForUserInfo helper function

Summary:
As I was attempting to introduce the use ens avatar option into EditAvatar I found that I needed the ethAddress again so I created this small helper function to reduce redundancy with this logic

Depends on D7460

Test Plan: flow and everything behaves as expected

Reviewers: ashoat, atul

Reviewed By: ashoat

Subscribers: ashoat, tomek, atul

Differential Revision: https://phab.comm.dev/D7461