Add convenience methods directly on IAsyncAction/Operation to facilitate future optimizations and custom Awaiter/Task types.

[jlaanstra]

This PR adds convenience methods for synchronously waiting/getting the result.

Additionally it exposes convenience methods for directly creating a completed/canceled/faulted IAsyncAction/Operation.

The underlying mechanics can be further optimized in future PRs by

• no longer requiring AsTask() for Wait()/Get()/GetAwaiter().
• creating more lightweight implementations of IAsyncAction/Operation without the logic related to wrapping Task/Task.