Test support: expose public constructor for sealed class used in ICodePackageActivationContext interface

[chuanboz]

While implementing microsoft/service-fabric-aspnetcore#48 and microsoft/service-fabric-aspnetcore#9 , I'm trying to provide a complete test that could fully cover the logic, I found that ICodePackageActivationContext interface has referenced many sealed class in System.Fabric that does not have public constructor.
Some examples are:
ConfigurationPackage
ConfigurationPackageDescription
ConfigurationSettings
ConfigurationSection
Right now one workaround that I see is to use Microsoft Fakes, that could work for .net framework but does not work for .net core.
It would be best if such class could provide a public constructor even making it sealed, so that in my test I could create instances of ICodePackageActivationContext and test the behavior.

this is copied from microsoft/service-fabric-aspnetcore#54

[masnider]

May also consider adding to https://github.com/loekd/ServiceFabric.Mocks