Enhancements to FAS key storage configuration

Currently, configuring where FAS private keys are stored is handled through the Citrix.Authentication.FederatedAuthenticationService.exe.config XML file as documented here. This has been a pain point for admins to manage and currently is not preserved over FAS upgrades.

With this change, configuration changes to how user and RA certificate private keys are stored will be handled separately, simplified, and preserved over upgrades.