Implement new Task API Contract

[baronfel]

The new Task should support modern .NET expectations, including but not limited to:

• async support (Task/ValueTask/etc)
• Cancellation (ctok parameter, managed/injected by the engine as part of the ExecuteAsync call)

The new Task should support a 'context' object parameter to ExecuteAsync that the engine can use to host APIs that are otherwise dangerous in a multi-threaded environment.

[AR-May]

I talked with @rainersigwald about the design for this new Task API and we think it is better to decouple this work into two separate designs:

1. thread-safe, context aware tasks
2. async tasks

We can easily decouple those if the 'context' object is not a parameter to the Execute function, but a property, being available to the thread-safe tasks, whether they are sync or async.

[yuehuang010]

FYI, the current Exec task uses 2-3 threads because stdout and stderr each uses a non-longrunning thread to pump messages. This can lead to thread starvation if it runs in the main msbuild process. The issue is the implementation of System.Diagnostics.Process.

This issue is ignore-able with multi-process msbuild but will be an issue for tasks in multi-threaded execution.