How do you use WithClientCertificate with secure websockets (wss) as a server?

### Describe your question
I have tried to use WithClientCertificate to always return true and output some message, but it doesn't appear as though the RemoteCertificateValidationCallback is being called at all:
```C#
public void ConfigureMqttServerOptions(MqttServerOptionsBuilder options)
{
options.WithConnectionValidator(this);
options.WithClientCertificate((sender, cert, chain, sslError) =>
{
Console.WriteLine("Validate certificate: {0}", sslError);
return true;
});
}
```
I am seeing output from the connection validator, however:
```C#
public Task ValidateConnectionAsync(MqttConnectionValidatorContext context)
{
Logger.LogInformation($"Opening connection [Secure={context.IsSecureConnection}]");

return Task.CompletedTask;
}
```

### Which project is your question related to?
- Server