Allow moving between transports

We should make it so that we can move back and forth between different transports/authenticators.

Right now, once you select an authenticator (scan the QR code, touch a USB security key, etc.), you have to finish the ceremony with that authenticator, or cancel the whole thing. I'd like the user to be able to start with a USB key, click the back button (e.g. because they realized it was the wrong one), select a different USB key, or select hybrid.

(I think scanning the hybrid QR code should probably lock you into finishing, but that's sort of a design question, and credentialsd doesn't need to make that decision; we can defer it to the UI.)

Inizia tracciando il flusso dello stato di selezione dell’autenticatore, del pulsante Indietro, della cancellazione e del trasporto descritto nell’issue. Confronta il passaggio da USB-key alla selezione di hybrid, quindi definisci le transizioni di stato e verifica che gli utenti possano tornare alla selezione e scegliere un altro autenticatore senza annullare la cerimonia.