Add SSL cert selector and validator callback support

With our usage of the .NET client, we came across the need to allow our code to select the client certificate being used and also to manually validate the remote SSL certificate.

This change allows the user code to set the SSL callbacks through the SslOption class that will be used by the SslHelper class.  If the user does not set either of these callbacks, then the default ones will be used, just like the previous version of the code.

I submitted the contributor agreement a while back, but haven't had a chance to send this change until now.