.NET: Update Workflow Input/Output Redesign (#881)

* feat: Make Executor id field mandatory

When checkpointing is involved, it is critical to keep executor ids consistent between runs, even when recreating a new object tree for the workflow.

The default id-setting mechanism generated a guid for part of the id, making it not work when restoring from a checkpoint.

This change prevents this situation from arising.

* feat: Enable running untyped Workflows

With the change to enable delay-instantiation of executors and support for async Executor factory methods, we must instantiate the starting executor to know what are the valid input types for the workflow.

To avoid forcing instantiation every time, and to better support workflows with multiple input types, we enable support for build and interacting with the base Workflow type without type annotations, and remove the requirement to know a valid input type when initiating a run.

* feat: Support Output from any executor and multiple outputs.

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

@@ -53,7 +53,7 @@ internal sealed class Program
 
         Stopwatch timer = Stopwatch.StartNew();
 
-        Workflow<string> workflow = this.CreateWorkflow();
+        Workflow workflow = this.CreateWorkflow();
 
         Notify($"\nWORKFLOW: Defined {timer.Elapsed}");
 
@@ -100,7 +100,7 @@ internal sealed class Program
         Notify("\nWORKFLOW: Done!\n");
     }
 
-    private Workflow<string> CreateWorkflow()
+    private Workflow CreateWorkflow()
     {
         // Use DeclarativeWorkflowBuilder to build a workflow based on a YAML file.
         DeclarativeWorkflowOptions options =