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.)

Comienza trazando el flujo de estados de selección del autenticador, botón de volver, cancelación y transporte descrito en el issue. Compara el cambio de USB-key con la selección de hybrid, define después las transiciones de estado y verifica que los usuarios puedan volver a la selección y elegir otro autenticador sin cancelar la ceremonia.