wasapi2: Add support for Win32 only build

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.
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