Runtime support for upcoming .NET Core WinRT Host (#23402)

Runtime support for upcoming .NET Core WinRT Host (#23402)

* First pass at adding winrt host entry-point.

* There's no way to specify HResult-swapping on a function called via hosted-interop so just return the HResult instead.

* Use the WindowsRuntimeMarshal class to create the activation factory and initialize it.

* Implement loading the dependent assemblies of a WinRT assembly into an isolated load context.

* PR Feedback.

* Fail to get the activation factory if the found type is not a managed type exported from a winmd.

* Rearrange parameters based on PR feedback.

* Remove unneeded include.

* Make ActivationFactoryLoader internal.

* Fix null-ref in WinRT-dependent-assembly loading

* Remove extraneous "System."