refactor: [BREAKING] Remove generic Workflow<T> (#1551)

Remove input type checking in favour of explicit `.DescribeProtocolAsync()` flow. Also removes `.AsAgentAsync()` as the validation happens at workflow run time. This makes it easier to use Workflows with DI without resorting to async-over-sync.

dotnet/samples/GettingStarted/Workflows/Checkpoint/CheckpointWithHumanInTheLoop/Program.cs

@@ -27,7 +27,7 @@ public static class Program
     private static async Task Main()
     {
         // Create the workflow
-        var workflow = await WorkflowHelper.GetWorkflowAsync();
+        var workflow = WorkflowHelper.GetWorkflow();
 
         // Create checkpoint manager
         var checkpointManager = CheckpointManager.Default;