Enforced async I/O when opening a SystemFile

[itsWindows11]

By default, the stream returned from File.Open does synchronous operations on the file. We can make the underlying operating system give us an asynchronous file stream to do true async reads/writes.

[Arlodotexe]

For our purposes, we should fix this because when synchronous (default) is set, there's no cancellation support on the async methods of FileStream. I hadn't noticed that before, maybe I need to add CommonTests for this.

Additionally, there's a lot of sync and async-specific optimizations done to improve performance, so many that they're using the strategy pattern in the dotnet implementation of FileStream. We should definitely expose this flag on SystemFile, but this can be done in a different ticket.