Contacts a backup provider and receives the backup from it.
This uses a QR code to contact another instance of deltachat which is providing a backup
BackupProvider. Once connected it will authenticate using the secrets in
the QR code and retrieve the backup.
This is a long running operation which will only when completed.
Qr as argument is a bit odd as it only accepts one specific variant of it. It
does avoid having [
iroh::provider::Ticket] in the primary API however, without
having to revert to untyped bytes.