This is part 1 of 3, split up of PR #23529. This PR refactors the
webauthn code to support passkey authentication/registration.
Passkeys aren't used yet, that is coming in PRs 2 and 3.
Co-authored-by: Alan Guo Xiang Tan <gxtan1990@gmail.com>
They're both constant per-instance values, there is no need to store them
in the session. This also makes the code a bit more readable by moving
the `session_challenge_key` method up to the `DiscourseWebauthn` module.
