The URL for Yubikey Validation server. OTP protocol specific
part will be appended to the ValidationServerURL. Defaults to
http://127.0.0.1:8003/yhsm/validate?
which is compatible
with Yubico's PyHSM validation server yhsm-val.
HTTPS requires
LWP::Protocol::https Perl module. If it's not installed, Radiator will log
an error without connecting to the server.
# Use Yubico's service over HTTPS
ValidationServerURL https://api.yubico.com/wsapi/2.0/verify?