This makes two changes:
- Adds a way to register a callback for when a FlutterDesktopPluginRegistrarRef is destroyed, and implements the logic to call it in the Windows and Linux embeddings.
- Adds a class to the C++ wrapper that handles making a singleton owning PluginRegistrar wrappers, and destroying them when the underlying reference goes away, to avoid needing that boilerplate code in every plugin's source.

Fixes https://github.com/flutter/flutter/issues/53496

This is a step toward aligning the API with macOS, and will make it easier to add the precompiled library later for release mode (since it can just be added to the project directory, without any code changes required for wrapper clients).

At the C API, uses a struct instead of individual arguments, mirroring a change that was already made on the Linux side to make the C API cleaner.

Functional changes in addition to the restructuring:

adds relative path support, as was recently added for GLFW
Uses wstring, rather than string, for paths; the conversion to UTF-8 is actually a potential problem on Windows, so pushing it into the embedding allows us the possibility of removing it later (if we can figure out a good solution at the embedder.h layer) without API breakage.
The old APIs used by the standard runner are left in place for now to avoid breaking the template on an engine roll. Once the framework template has been updated, the old API paths will be removed.

Clang has different warning settings, so catches different issues than
the VS compile. This fixes various minor issues caught by clang.

Part of https://github.com/flutter/flutter/issues/16256

plugin_registrar_windows.h was never fully updated for the Win32 switch,
and didn't actually compile. This introduces FlutterView (parallel to
the GLFW wrapper's FlutterWindow) as a ways of holding view-specific
functionality to expose via the plugin registrar, and moves HWND access
from the FlutterViewController to the FlutterView so that it's
available to plugins. This allows the implementation of plugins that need
access to the native HWND (e.g., moving or resizing the top-level window).

Adds simple unit tests of the new wrapper functionality, ensuring that the
files actually compile, and that the passthroughs work as expected.

This is a breaking change for Windows runners due to moving
GetNativeWindow() in the wrapper. It's not being done as a multi-stage
change (addition + deprecation + later removal) since this API is explicitly
unstable.

Adds a task runner, and exposes API to allow application-level runloops to know when they need to next call the API to process engine events. Internally, sends null events to wake up the app runloop when new events are scheduled to ensure the wait time is updated accordingly.

Fixes #36420

Add a context for async resource uploading.

Fixes flutter/flutter#36422

* Begin API evolution to a more native win32 API

* Child-window based hosting

* Plumb through an initial size for child window to avoid reallocated surface on start

* Windows API cleanup part 1

* Fix wrapper tests

* Ensure flutter's HWND resources are destroyed

* Final API cleanup

* Fix dynamic DPI handling

* Cleanup

* Fix a bug that was causing engine to not be shutdown correctly

* CR feedback

* auto format

* CR feedback: combine FlutterView and FlutterViewController

* The one that clang-format seems to always get wrong

* expletive

* fix sources for licesnse file

* CR Feedback

* cleanup

* Update GetNativeWindow() to return an HWND rather than a long

* fix formatting

Start work on flutter/flutter#30726 by adding an alternative win32 shell platform implementation for Windows that is not based on GLFW and that uses LIBANGLE for rendering and native win32 windowing and input. This change does not replace the GLFW implementation but rather runs side by side with it producing a secondary flutter_windows_win32.dll artifact. The following items must be added to attain parity with the GLFW implementation:
- Custom task scheduling
- Support for keyboard modifier keys
- Async texture uploads
- Correct high DPI handling on Windows versions < 1703
and will be added in subsequent changes.