Export support for PS class-based resources

[anmenaga]

PR Summary

Fix #183

2 updates: 1) Adds Export support for provider-based resources. It is done using export node in *.dsc.resource.json file similar to get/set/test. 2) Adds Export support to class-based PS resources. To support this a DscResource class has to implement a static Export function like so:

using namespace System.Collections.Generic

[DscResource()]
class TestClassResource
{
# omitting properties and Get,Set,Test methods here

    static [TestClassResource[]] Export()
    {
        $resultList = [List[TestClassResource]]::new()
        1..5 | %{
            $obj = New-Object TestClassResource
            $obj.Name = "Object$_"
            $obj.Prop1 = "Property of object$_"
            $resultList.Add($obj)
        }

        return $resultList.ToArray()
    }
}

[anmenaga]

This design will allow class based resources implementing export to still work with DSC v2, right?

Yes.

v2 should just ignore the new export method?

Correct.

Ideally we should have a test to ensure we aren't breaking compat.

We already test that implicitly. We are using v2 throughout PowerShellGroup resource tests.