Support FIDO2 user verification

Go to https://webauthn.io/

Put any username, click on "Advanced settings" and set "User verification" to "Required".

Now click "Register".

In Chromium, a popup shows up asking for PIN of YubiKey (I'm using YubiKey 5C Nano):

In qutebrowser for example, which is based on QtWebEngine, this will not work, because there is way to enter a PIN.

So either QtWebEngine needs to display such a dialog itself, or it needs to provide an API for applications to do so.

Thanks by the way for implementing WebAuthn support in QtWebEngine!