.Net: Simplify example 52 to showcase using a custom OpenAIClient in …

…genera… (#4502)

…l instead of using one specifically for APIM

### Motivation and Context
Setting up APIM and AAD is tedious and is not directly related to what
we want to showcase: that we can customize OpenAIClient instances used
with SK.

resolves #3844 

### Description
Simplify examples and its requirements.

### Contribution Checklist
- [ ] The code builds clean without any errors or warnings
- [ ] The PR follows the [SK Contribution
Guidelines](https://github.com/microsoft/semantic-kernel/blob/main/CONTRIBUTING.md)
and the [pre-submission formatting
script](https://github.com/microsoft/semantic-kernel/blob/main/CONTRIBUTING.md#development-scripts)
raises no violations
- [ ] All unit tests pass, and I have added new tests where possible
- [ ] I didn't break anyone 😄

---------

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

dotnet/samples/KernelSyntaxExamples/Example52_CustomOpenAIClient.cs

@@ -0,0 +1,58 @@
+// Copyright (c) Microsoft. All rights reserved.
+
+using System;
+using System.IO;
+using System.Net.Http;
+using System.Threading.Tasks;
+using Azure;
+using Azure.AI.OpenAI;
+using Azure.Core.Pipeline;
+using Microsoft.SemanticKernel;
+using RepoUtils;
+
+public static class Example52_CustomOpenAIClient
+{
+    public static async Task RunAsync()
+    {
+        Console.WriteLine("======== Using a custom OpenAI client ========");
+
+        string endpoint = TestConfiguration.AzureOpenAI.Endpoint;
+        string deploymentName = TestConfiguration.AzureOpenAI.ChatDeploymentName;
+        string apiKey = TestConfiguration.AzureOpenAI.ApiKey;
+
+        if (endpoint is null || deploymentName is null || apiKey is null)
+        {
+            Console.WriteLine("Azure OpenAI credentials not found. Skipping example.");
+            return;
+        }
+
+        // Create an HttpClient and include your custom header(s)
+        var httpClient = new HttpClient();
+        httpClient.DefaultRequestHeaders.Add("x-my-custom-header", "My custom value");
+
+        // Configure OpenAIClient to use the customized HttpClient
+        var clientOptions = new OpenAIClientOptions
+        {
+            Transport = new HttpClientTransport(httpClient),
+        };
+        var openAIClient = new OpenAIClient(new Uri(endpoint), new AzureKeyCredential(apiKey), clientOptions);
+
+        IKernelBuilder builder = Kernel.CreateBuilder();
+        builder.AddAzureOpenAIChatCompletion(deploymentName, openAIClient);
+        Kernel kernel = builder.Build();
+
+        // Load semantic plugin defined with prompt templates
+        string folder = RepoFiles.SamplePluginsPath();
+
+        kernel.ImportPluginFromPromptDirectory(Path.Combine(folder, "FunPlugin"));
+
+        // Run
+        var result = await kernel.InvokeAsync(
+            kernel.Plugins["FunPlugin"]["Excuses"],
+            new() { ["input"] = "I have no homework" }
+        );
+        Console.WriteLine(result.GetValue<string>());
+
+        httpClient.Dispose();
+    }
+}

dotnet/samples/KernelSyntaxExamples/README.md

@@ -103,9 +103,6 @@ dotnet user-secrets set "Google:SearchEngineId" "..."
 
 dotnet user-secrets set "Github:PAT" "github_pat_..."
 
-dotnet user-secrets set "Apim:Endpoint" "https://apim...azure-api.net/"
-dotnet user-secrets set "Apim:SubscriptionKey" "..."
-
 dotnet user-secrets set "Postgres:ConnectionString" "..."
 dotnet user-secrets set "Redis:Configuration" "..."
 dotnet user-secrets set "Kusto:ConnectionString" "..."
@@ -173,10 +170,6 @@ Google__SearchEngineId
 # Github
 Github__PAT
 
-# Azure API Management (APIM)
-Apim__Endpoint
-Apim__SubscriptionKey
-
 # Other
 Postgres__ConnectionString
 Redis__Configuration