This adds support for the `secure_backup_setup_methods` key, which allows HS
admins to state that Element should simplify down to only one setup method,
rather than offering both.
Fixes https://github.com/vector-im/element-web/issues/15238
With this change, Element now creates cross-signing keys during auth flows for
password login. For other auth flows like token / SSO, it will not happen until
a cross-signing / secret storage dialog flow as before.