Create metamask user-name registrar service

We should grab a friendly ENS domain, and readily offer its subdomains to our users, to help coherent nicknames to become common.

Not posting our candidates here to avoid bid sniping.

The issue proposes an ENS domain and a registrar service for offering subdomains, but names no files, tests, or entry points. Start by clarifying the domain choice, registration workflow, user ownership, and service boundaries; done requires an agreed design and an implemented way to provide coherent user subdomains.

Written by the indexing model from the issue text.