.NET: Add DevUI package for .NET (#1603)

* Implement DevUI

* Review feedback

* Fix build

dotnet/samples/GettingStarted/DevUI/README.md

@@ -0,0 +1,57 @@
+# DevUI Samples
+
+This folder contains samples demonstrating how to use the DevUI in ASP.NET Core applications.
+
+## What is DevUI?
+
+The DevUI provides an interactive web interface for testing and debugging AI agents during development.
+
+## Samples
+
+### [DevUI_Step01_BasicUsage](./DevUI_Step01_BasicUsage)
+
+Shows how to add DevUI to an ASP.NET Core application with multiple agents and workflows.
+
+**Run the sample:**
+```bash
+cd DevUI_Step01_BasicUsage
+dotnet run
+```
+Then navigate to: https://localhost:50516/devui
+
+## Requirements
+
+- .NET 8.0 or later
+- ASP.NET Core
+- Azure OpenAI credentials
+
+## Quick Start
+
+To add DevUI to your application:
+
+```csharp
+var builder = WebApplication.CreateBuilder(args);
+
+// Set up the chat client
+builder.Services.AddChatClient(chatClient);
+
+// Register your agents
+builder.AddAIAgent("my-agent", "You are a helpful assistant.");
+
+// Add DevUI services
+builder.AddDevUI();
+
+var app = builder.Build();
+
+// Map the DevUI endpoint
+app.MapDevUI();
+
+// Add required endpoints
+app.MapEntities();
+app.MapOpenAIResponses();
+app.MapOpenAIConversations();
+
+app.Run();
+```
+
+Then navigate to `/devui` in your browser.