Add Get-SitecoreCommercePolicy

[richardszalay]

The "filesource" parameter set for Set-SitecoreCommercePolicy was removed as it added unnecessary complexity, but it could easily be restored by adding a Get-SitecoreCommercePolicy.

Usecases would be:

# Merge policies and their properties from another policy file
Get-SitecoreCommercePolicy -Path "source.json" | Set-SItecoreCommercePolicy -Path "target.json"

# Add policies if they don't exist on the target
Get-SitecoreCommercePolicy -Path "source.json" | Set-SItecoreCommercePolicy -Path "target.json" -Force

(The pipelining isn't currently supported by Set, so that change will be required too)