Implement io_uring support for FileStream

We have recently invested a lot of time in rewriting FileStream on Windows. We have kept io_uring in mind and after recent refactoring, it should be now much easier to implement the support:

• we have introduced a new internal abstraction called FileStreamStrategy. It's more or less FileStream API.
• FileStream can choose the strategy at runtime. In the case of Linux, it could detect the kernel version and just use the new strategy for newer kernels (5.5+). It means that the day our customers update their kernel version, .NET could start using io_uring without a .NET update.
• Entire buffering logic has been moved to a new strategy called BufferedFileStreamStrategy which can be used as a wrapper over another strategy. It means that new strategies (like IoUringStrategy) don't need to worry about buffering at all https://github.com/dotnet/runtime/blob/2223babdd49118787c675e04aff711f936a10b26/src/libraries/System.Private.CoreLib/src/System/IO/Strategies/FileStreamHelpers.Windows.cs#L59-L60
• We can use the existing Unix strategy for sync file IO, so the new IoUringStrategy would only need to implement ReadAsync and WriteAsync support.

We (owners of System.IO) have a lot of other high-priority things on our schedule for .NET 6 (like full symbolic links support) and since most of our customers are not using the latest Linux kernels, we are most probably won't be able to implement it on our own for .NET 6. But we would love to provide any help necessary (code reviews, testing) for a contributor that would be willing to implement it. Having said that, I am marking this issue as "up-for-grabs".

If we won't find a contributor for .NET 6, we are going to include this in .NET 7 planning and deliver it in .NET 7.