Add application certificate to PINGID request with SAML 2.0 auth

[RoelEsselink]

Add application certificate to PINGID request from Alteryx as part of a mutual app authentication process for SAML 2.0. At the moment when enabling SAML 2.0 for Alteryx there is not application certificate being sent with the authorisation request. Request is to add this. The risk that request without certificate will be abused is very low but still we would like mitigate this small risk by adding an application certificate.