Handle login with multiple keys configured

[Natim]

Now we are just trying the first key, but the goal would be to try the assertion until one works.

[Natim]

The Python side looks good, we need to call the verify_assertion for each keys on the client side.

Also maybe, we want to create a different challenge for each key. We can do that by prefixing the session challenge key with the credential_id of the webauthn_key

session[f"challenge_{credential_id}"]