Allow mocking of PowerShell object in `PowerShellServices` class

[davidmrdavid]

Related: #839 (comment)

This will make it easier to test some of the external DF SDK logic that has been implemented in PowerShellServices.cs. Being able to unit test the method isExternalDurableSDKLoaded will be a requirement prior to GA'ing this new experience.

[AnatoliB]

@davidmrdavid I have to note that the original intent of PowerShellServices was exactly that: enable unit-testing without having to instantiate an actual PowerShell object. PowerShellServices was supposed to be a very thin layer with little to no logic except for forwarding calls to the real PowerShell. Having said that, "original" does not mean "must stay this way forever", but consider following this idea, moving the isExternalDurableSdkLoaded method somewhere else, and making it rely on IPowerShellServices (very easy to mock) instead of PowerShell. Consider doing this with the (now relatively complex) logic in SetOrchestrationContext as well.