Switches the Windows embedding from the standard C API to the new proctable version, to allow for unit testing of the embedding layer separately from the embedder APIs implementation. This includes moving some engine messaging that was still in flutter_windows to the C++ engine class to better encapsulate the proc table.
Replaces the (temporary) compile-time option to pass engine switches
with the ability to pass them temporarily at runtime via environment
variables.
This is enabled only for debug/profile to avoid potential issues with
tampering with released applicaitons, but if there is a need for that in
the future it could be added (potentially with a whitelist, as is
currently used for Dart VM flags).
Windows portion of:
https://github.com/flutter/flutter/issues/38569https://github.com/flutter/flutter/issues/60393
Refactors the Windows embedding internals to make an engine object that
owns things associated with the engine rather than the view, and updates
the API surface to allow using the engine directly.
This is an incremental step toward both a cleaner, non-struct-based
internal structure and a finalized API surface.
