Fix issue #428 dsc firewall

[brajjan]

Pull Request (PR) description

Added PolicyStore parameter and read only property PolicyStoreSourceType - fixes Issue #428. As Get-NetFirewallRule does not return the PolicyStore property I added the PolicyStoreSourceType as a read only property in the MOF file, added it to DSC_Firewall.data.psd1 as well as a parameter for Test-TargetResource and Set-TargetResource for comparing if it is in desired state.

• Parameter PolicyStore set to localhost returns PolicyStoreSourceType: GPO
• Parameter PolicyStore set to PersistentStore returns PolicyStoreSourceType: Local

Not updated Pester tests as my knowledge of Pester tests are close to 0

This Pull Request (PR) fixes the following issues

• Fixes Firewall and FirewallProfile: Add PolicyStore to allow targeting local group policy #428

Task list

• Added an entry to the change log under the Unreleased section of the
  file CHANGELOG.md. Entry should say what was changed and how that
  affects users (if applicable), and reference the issue being resolved
  (if applicable).
• Resource documentation added/updated in README.md.
• Resource parameter descriptions added/updated in README.md, schema.mof
  and comment-based help.
• Comment-based help added/updated.
• Localization strings added/updated in all localization files as appropriate.
• Examples appropriately added/updated.
• Unit tests added/updated. See DSC Community Testing Guidelines.
• Integration tests added/updated (where possible). See DSC Community Testing Guidelines.
• New/changed code adheres to DSC Community Style Guidelines.

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This change is