Support Win32 Events

[qmfrederik]

Overlapped I/O uses Win32 events to signal the completion of an overlapped I/O operation. .NET nor PInvoke.Kernel32 provide built-in support for working with Win32 events.

Consider adding support for:

• CreateEventEx, SetEvent and ResetEvent. There are P/Invoke declarations in .NET Core
• Asynchronously awaiting events. Microsoft.VisualStudio.Threading.TplExtensions.ToTask provides a reference implementation.

Split off from https://github.com/dotnet/pinvoke/pull/513#discussion_r456068538

[AArnott]

The p/invokes are fine to add. I'm less convinced we should create and/or expose an GetAwaiter extension for WaitHandle, particularly since it's already done in a public Microsoft-owned package. If we needed a copy internally, we could consider that.