This adds various customisations point in the app for security related
decisions. By default, these do nothing, but would be customised at the
app level via module replacement (so that no changes are needed here in the
SDK).
Fixes https://github.com/vector-im/element-web/issues/15350
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.
