Support FIDO2 WebAuthn 2FA

[uumas]

Current behavior:
When trying to login to an account with FIDO2 WebAuthn as the 2fa method, rbw returns rbw login: TODO

[quexten]

#116 implements this, but is waiting on webauthn-rs to release a new stable version. But you can have this right now if you compile the PR version locally.

[HenkVanMaanen]

@quexten it seems like webauthn-rs 0.5.0 became stable last month .

[quexten]

Thanks for the ping. It seems they changed the API for how to access fido2 tokens, requiring (at least a small) re-write. TBH I'm not an rbw user anymore, so I'm not sure whether I'll continue the PR. If I get to writing a rust based implementation for fido2, for the upstream desktop clients, I can look into applying that work to this PR though. Otherwise, feel anyone reading this, feel free to take over the PR.