Make storage extensions conditional

[Scottj1s]

To support Windows interop work. BufferedStream.BufferSize and BufferedStream.UnderlyingStream require .NET Standard 2.1

[manodasanW]

How about that one?

private static object AsWindowsRuntimeStreamInternal(Stream stream)
{
// Check to see if the managed stream is actually a wrapper of a WinRT stream:
// (This can be either an adapter directly, or an adapter wrapped in a BufferedStream.)
WinRtToNetFxStreamAdapter sAdptr = stream as WinRtToNetFxStreamAdapter;
if (sAdptr == null)
{
BufferedStream buffAdptr = stream as BufferedStream;
if (buffAdptr != null)
sAdptr = buffAdptr.UnderlyingStream as WinRtToNetFxStreamAdapter;
}

[Scottj1s]

AsWindowsRuntimeStreamInternal

good catch