authz: Sync user permissions based on external service tokens

Created by: ryanslade

This change modifies our permissions syncer so that in addition to syncing user permissions using external accounts (sign in connections) it will now also sync using OAuth tokens in user added code hosts (code host connections).

This allows us to properly sync user permissions for repos that have only been added by a user added external service and allows us to remove a workaround that we added to the repo syncer: https://github.com/sourcegraph/sourcegraph/pull/20727

The repo syncer workaround will be removed in subsequent PR.