Add more granular settings for policy export

[StefanIvemo]

Describe the solution you'd like

I would like to get the possibility to do granular filtering on what policy resources to export. Today we only have the option to configure SkipPolicy: true/false which includes exemptions, assignments, definitions and policysetdefinitions. I have a case where the customer only wants to export some of these and there is no way to filter out just a few of them. Adding for example Microsoft.Authorization/policyExemptions to Core.SkipResourceType will not work.

Suggested solutions:

1. Change the logic of policy export to honor any resource types specified in Core.SkipResourceType.
2. Add additional properties to settings.json. E.g. Core.SkipPolicyType.

"Core.SkipPolicyType": [
     "policyexemptions",
     "policyassignments"
],