Support `--what-if` for built-in resources

[Gijsreyn]

Summary of the new feature / enhancement

As a user,
I want to be able to call --what-if on built-in resources,
So I can preview the changes it is going to apply.

Proposed technical implementation details (optional)

With DSC v3.2 shipped, a couple of new built-in resources are introduced with get, set, and export. But these built-in resources are perfect candidates to implement/showcase --what-if:

• Microsoft.Windows/FirewallRuleList
• Microsoft.Windows/Service
• Microsoft.Windows/FeatureOnDemandList
• Microsoft.Windows/OptionalFeatureList
• Microsoft.Windows/UpdateList
• Microsoft.OpenSSH.SSHD/sshd_config
• Microsoft.OpenSSH.SSHD/Subsystem
• Microsoft.OpenSSH.SSHD/SubsystemList
• Microsoft.OpenSSH.SSHD/Windows