[keyserver] Exclude ETH accounts from username reservation system

Summary:
We should not add ETH addresses to the reserved usernames list on the identity service

Resolves https://linear.app/comm/issue/ENG-4335/make-sure-we-exclude-eth-accounts-from-username-reservation-system

Test Plan: ran the cronjob locally and saw that the ETH usernames in my users table weren't added to the DDB table

Reviewers: ashoat, atul

Reviewed By: ashoat

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D8481