The only difference between Win32 and WinRT (including UWP) is device enumeration and activation. Split that part for Win32 and WinRT so that wasapi2 plugin can support old OS (e.g., Windows 7) and MinGW build as well.
In case that both Win32 and WinRT are available, user can select target implementation via "GST_USE_WASAPI2_WINRT_CLIENT" env var, which is conceptually equivalent to "GST_USE_MF_WINRT_CAPTURE" env of MediaFoundation plugin.