.Net: Add API to create a PromptTemplateConfig from a Prompty template

[markwallace-microsoft]

Motivation and Context

Adding this API allows developers to write code which creates an IPromptTemplate instance from a Prompty template so that it can be used with any IAIService implementation.

This change enables the following flow:

      Kernel kernel = Kernel.CreateBuilder()
          .AddOpenAIChatCompletion(
              modelId: TestConfiguration.OpenAI.ChatModelId,
              apiKey: TestConfiguration.OpenAI.ApiKey)
          .Build();

      string promptyTemplate = """
          ---
          name: Contoso_Prompt
          description: A sample prompt that responds with what Seattle is.
          authors:
            - ????
          model:
            api: chat
          ---
          What is Seattle?
          """;

      var promptConfig = PromptyKernelExtensions.ToPromptTemplateConfig(promptyTemplate);
      var promptTemplateFactory = new LiquidPromptTemplateFactory();
      var promptTemplate = promptTemplateFactory.Create(promptConfig);
      var prompt = await promptTemplate.RenderAsync(kernel);

      var chatService = kernel.GetRequiredService<IChatCompletionService>();
      var result = await chatService.GetChatMessageContentAsync(prompt);

Description

Contribution Checklist

• The code builds clean without any errors or warnings
• The PR follows the SK Contribution Guidelines and the pre-submission formatting script raises no violations
• All unit tests pass, and I have added new tests where possible
• I didn't break anyone 😄

[markwallace-microsoft]

FYI @LittleLittleCloud

[RogerBarreto]

Add this Example in the README.md from Concepts