Add support for imported keys on yubikey kms

[maraino]

Description

step-ca or step-kms-plugin fails to sign with a key imported into the yubikey. The cause of this is because we attest the slot to get the public key. We also support the workaround of getting the certificate in the same slot. But we can easily support imported keys without the workaround if we use KeyInfo to get the public key, it would be something something like this:

func (k *YubiKey) getPublicKey(slot piv.Slot) (crypto.PublicKey, error) {
	ki, err := k.yk.KeyInfo(slot)
	if err != nil {
		return nil, err
	}
	// not use if this can be nil
	if ki.PublicKey == nil {
		 return nil, errors.New("error retrieving public key")
	}
	return ki.PublicKey, nil
}

KeyInfo is only supported on YubiKeys with a version >= 5.3.0, so we might need to fallback to Attest and Certificate.