Add 'CoCreateInstance' helper

[Sergio0694]

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This PR adds the following APIs:

namespace WinRT;

public enum ClassContext : uint
{
    InProcServer,
    LocalServer
}

public class IInspectable
{
    public static T CoCreateInstance<T>(in Guid classId, in Guid interfaceId, ClassContext classContext)
        where T : class;
}

These can be used to easily create WinRT objects from out-of-proc COM servers.

For instance, for a Windows Package Manager type:

PackageManager packageManager = IInspectable.CoCreateInstance<PackageManager>(
    CLSID_PackageManager,
    IID_IPackageManager,
    ClassContext.LocalServer);

[shmuelie]

As the IID is in the metadata for types, could we reference it that way instead of needing to declare it explicitly? (Sadly not true for CLSID)

[Sergio0694]

Closing this, we're not really planning on adding new functionality to 2.x at this point. We can re-evaluate in the future whether we'd want a public API like this in 3.0, though the API surface and implementation would have to be different for sure.