Windows plugins are designed to share implementations between Win32 and
UWP, but not all plugins will support both. This adds a new
'supportedVariants' key to Windows plugins that allows specifying
'win32' and/or 'uwp' (and potentially others in the future in case that
becomes necessary).

Plugins without any supported variants will be assumed to be Win32 for
backward compatibility.

This will allow compiling Windows projects that use Win32-only Windows
plugins (which is currently all of them) in UWP mode. The plugins will
of course throw missing implementation exceptions at runtime, but tehy
won't prevent being able to build as they currently do.

Fixes https://github.com/flutter/flutter/issues/82815