Support passwordless login with Passkeys

[Falco]

@Saklad5

The only change Discourse needs to make to support them properly is allowing a security key to be used instead of a password, rather than as a form of two-factor authentication.

That was covered in our original spec for webauthn

@featheredtoast

Authentication Methods

• Webauthn as a second factor authenticator (act like a Google Authenticator alternative)
• Webauthn as a first factor authenticator (act like a social login alternative)
• Webauthn as a multi factor authenticator (username-less login)

However we only implemented the first, and most common webauthn method.

@sam

I would very much like to avoid even thinking about “First factor / passwordless” auth here, to me we got to ship this feature and live with it for 3-4 months before even considering this.