This adds persistence to the Nextcloud server 2FA logic so that the server
knows which 2FA providers are enabled for a specific user at any time, even
when the provider is not available.
The `IStatefulProvider` interface was added as tagging interface for providers
that are compatible with this new API.
Signed-off-by: Christoph Wurst <christoph@winzerhof-wurst.at>
Christoph Wurst