.NET: Add support for Subworkflows and many threading fixes (#1066)

* feat: Add support for Workflow-as-Executor

* Fixes routing of 'object' compile-typed variables to properly take in type information
* Fixes a concurrency issue in StepTracer

* fix: Make Subworkflow ExternalRequests work properly

* fix: Threading and Concurrency fixes; prep for OffThread Mode

* refactor: Remove dead code around OffStreamRunEventStream

Currently not used, and will be replaced with a rewrite when brought back, so having it in the change is not valuable.

* ci: Work around issues with dotnet-format not properly analyzing the source

* fix: Fix the logic of AsyncCoordinator and AsyncBarrier

* Prevent individual wait cancellations from canceling the entire barrier
* Propagate information about whether the wait was completed or cancelled, and whether any waiters were present when released

* fix: Remove superfluous acces to .Keys in InProcStepTracer

* refactor: Clean up AsyncCoordinator's use of AsyncBarrier

dotnet/src/Microsoft.Agents.AI.Workflows/WorkflowErrorEvent.cs

@@ -10,4 +10,4 @@ namespace Microsoft.Agents.AI.Workflows;
 /// <param name="e">
 /// Optionally, the <see cref="Exception"/> representing the error.
 /// </param>
-public sealed class WorkflowErrorEvent(Exception? e) : WorkflowEvent(e);
+public class WorkflowErrorEvent(Exception? e) : WorkflowEvent(e);