Apply policy rules based on scope

Policy assignments are define against a specific scope. Currently if policy as rules are exported from Azure Policy all rules will apply to matching resource regardless of resource scope.

To support resource scope policy rules should use a selector or sub-selector to apply to:

- Azure resources with no scope. (optional?)
- Azure resources within the policy scope (minus exclusions and exemptions).

PSRule introduced support for a `scope` property with microsoft/PSRule#1350.