.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/SubworkflowWarningEvent.cs

@@ -0,0 +1,17 @@
+// Copyright (c) Microsoft. All rights reserved.
+
+namespace Microsoft.Agents.AI.Workflows;
+
+/// <summary>
+/// Event triggered when a subworkflow encounters a warning-confition.
+/// sub-workflow.
+/// </summary>
+/// <param name="message">The warning message.</param>
+/// <param name="subWorkflowId">The unique identifier of the sub-workflow that triggered the warning. Cannot be null or empty.</param>
+public sealed class SubworkflowWarningEvent(string message, string subWorkflowId) : WorkflowWarningEvent(message)
+{
+    /// <summary>
+    /// The unique identifier of the sub-workflow that triggered the warning.
+    /// </summary>
+    public string SubWorkflowId { get; } = subWorkflowId;
+}