.Net: Add custom PromptConfig to Stepwise Planner (#2000)

### Motivation and Context

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In case of custom prompt, allow sending custom prompt config. This is
useful for custom prompt to Stepwise Planner having additional
parameters.

### Description

<!-- Describe your changes, the overall approach, the underlying design.
These notes will help understanding how your code works. Thanks! -->

This allow stepwise planner to be created with custom prompt and custom
prompt config

### Contribution Checklist

<!-- Before submitting this PR, please make sure: -->

- [x] The code builds clean without any errors or warnings
- [x] 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#dev-scripts)
raises no violations
- [x] All unit tests pass, and I have added new tests where possible
- [x] I didn't break anyone 😄

---------

Co-authored-by: Shawn Callegari <36091529+shawncal@users.noreply.github.com>

dotnet/src/Extensions/Planning.StepwisePlanner/StepwisePlanner.cs

@@ -34,23 +34,29 @@ public class StepwisePlanner
     /// <param name="kernel">The semantic kernel instance.</param>
     /// <param name="config">Optional configuration object</param>
     /// <param name="prompt">Optional prompt override</param>
+    /// <param name="promptUserConfig">Optional prompt config override</param>
     public StepwisePlanner(
         IKernel kernel,
         StepwisePlannerConfig? config = null,
-        string? prompt = null)
+        string? prompt = null,
+        PromptTemplateConfig? promptUserConfig = null)
     {
         Verify.NotNull(kernel);
         this._kernel = kernel;
 
         this.Config = config ?? new();
         this.Config.ExcludedSkills.Add(RestrictedSkillName);
 
-        var promptConfig = new PromptTemplateConfig();
+        var promptConfig = promptUserConfig ?? new PromptTemplateConfig();
         var promptTemplate = prompt ?? EmbeddedResource.Read("Skills.StepwiseStep.skprompt.txt");
-        string promptConfigString = EmbeddedResource.Read("Skills.StepwiseStep.config.json");
-        if (!string.IsNullOrEmpty(promptConfigString))
+
+        if (promptUserConfig == null)
         {
-            promptConfig = PromptTemplateConfig.FromJson(promptConfigString);
+            string promptConfigString = EmbeddedResource.Read("Skills.StepwiseStep.config.json");
+            if (!string.IsNullOrEmpty(promptConfigString))
+            {
+                promptConfig = PromptTemplateConfig.FromJson(promptConfigString);
+            }
         }
 
         promptConfig.Completion.MaxTokens = this.Config.MaxTokens;