[Windows] Allow view controllers to not own the engine (#51570)

Adds `FlutterDesktopEngineCreateViewController` to the Windows embedder's C API. Example usage:

```cpp
// Create the engine.
FlutterDesktopEngineProperties engine_properties = {};
auto engine = FlutterDesktopEngineCreate(&engine_properties);

// Create a views controller.
FlutterDesktopViewControllerProperties properties = {};
properties.width = 400;
properties.height = 400;

auto controller = FlutterDesktopEngineCreateViewController(engine, &properties);

// Run app...

// Destroy the view controller. This does not destroy the engine. 
FlutterDesktopViewControllerDestroy(controller);

// Destroy the engine.
FlutterDesktopEngineDestroy(engine);
```

This new API creates a view controller that does **not** own the engine, unlike the existing `FlutterDesktopEngineCreateViewController`. In the future, this new API will allow a Windows app to have multiple views that share a single engine.

This API is in the `flutter_windows_internal.h`, making it "pubternal". This API is not in the public Windows header files, however, one can link against this API for testing purposes. This API will be promoted to the public Windows header files once the Windows embedder is ready to have multiple views.

Relevant design documents:
1. https://flutter.dev/go/desktop-multi-view-runner-apis
1. http://flutter.dev/go/windows-multi-view-ownership-updates

Relevant issues:
1. https://github.com/flutter/flutter/issues/143767
1. https://github.com/flutter/flutter/issues/142845

[C++, Objective-C, Java style guides]: https://github.com/flutter/engine/blob/main/CONTRIBUTING.md#style