.NET: [BREAKING] Support Checkpoint Serialization (#735)

* feat: Support Checkpoint Serialization

* Implements serialization roundtripping for checkpoints.
* Adds support for JSON serialization
* Adds FileSystem-based checkpoint persistence

* fix: Executor State does not deserialize correctly

The StateManager was not properly handling delay-deserialized values.

* Fix PortableValue handling in StateManager (this makes it delegate to PortableValue the uwnrapping)
* Fix UnitTest to actually test checkpoint serialization
* Additional review comment fixes

---------

Co-authored-by: Chris <66376200+crickman@users.noreply.github.com>

dotnet/samples/GettingStarted/Workflows/Declarative/Program.cs

@@ -53,7 +53,7 @@ internal sealed class Program
         // Run the workflow, just like any other workflow
         string input = this.GetWorkflowInput();
 
-        CheckpointManager checkpointManager = new();
+        CheckpointManager checkpointManager = CheckpointManager.Default;
         Checkpointed<StreamingRun> run = await InProcessExecution.StreamAsync(workflow, input, checkpointManager);
 
         bool isComplete = false;
@@ -151,7 +151,7 @@ internal sealed class Program
                     Debug.WriteLine($"ACTION EXIT #{actionComplete.ActionId} [{actionComplete.ActionType}]");
                     break;
 
-                case ExecutorFailureEvent executorFailure:
+                case ExecutorFailedEvent executorFailure:
                     Debug.WriteLine($"STEP ERROR #{executorFailure.ExecutorId}: {executorFailure.Data?.Message ?? "Unknown"}");
                     break;
 
@@ -256,7 +256,7 @@ internal sealed class Program
     }
     private static InputResponse HandleExternalRequest(ExternalRequest request)
     {
-        InputRequest? message = request.Data as InputRequest;
+        InputRequest? message = request.Data.As<InputRequest>();
         string? userInput = null;
         do
         {