Access hubs.mozilla.com account from other hubs-cloud instances

I'd like to have the account in HMC act as a kind of master account for other instances. I don't want my content from my HMC account to show up automatically, but when I log into a hubs-cloud instance, it would be nice if there was a way to browse the avatars I use for HMC and pull them to the new account; or just "link them", so if I upload avatars on the hubs-cloud instance, they get copied to the HMC account too.

Obviously, this needs to be an explicit choice.

Alternatively, being able to use another hubs-cloud with the same email, or even specify the email and cloud (which would, of course, then require some form of authentication).

┆Issue is synchronized with this [Jira Task](https://mozilla-hub.atlassian.net/browse/HUBS-1032)

No files, tests, or entry points are identified in the issue. Start by locating the account and avatar authentication flows across Hubs Cloud instances; done should include explicit cross-instance account linking or authenticated access without automatically exposing HMC content.

Written by the indexing model from the issue text.