.Net: Markdown prompt support (without tool call support)

[LittleLittleCloud]

Motivation and Context

Description

This PR brings markdown prompt template support for semantic kernel. It essentially does a code-level copy of the original markdown prompt template implementation to the sk repo, plus an extension API to execute the template.

Because the markdown prompt template support is still in an very, very early stage. All code is put under Experimental namespace and all the class except the extension API is marked as internal only.

You can find the spec for the markdown prompt template here

The intergration comes with two projects

• PromptTemplates.Liquid: liquid syntax support for the markdown prompt template, which renders liquid-like template into the chat format that can be processed by ChatPromptParser
• Function.Prompty: load and create KernelFunctionFromPrompt from prompty file via CreateFunctionFromPrompty API

The tool call support will come in the next PR as this PR is already growing large. Also tool call support needs some extra care which might need further discussion on how to implement cohere to sk pattern

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 😄

[stephentoub]

Is this information really put into a prompty file?

[markwallace-microsoft]

This information would tie a Prompty file to one particular deployment. These will change frequently as new models emerge and these artifacts should be easily transferable to other contexts e.g. a Prompty file the Data Scientists developed should be transferable to another team to deploy. Or if I'm building a product my Prompty files should run with any customer deployment.
In our YAML file we include the model configuration i.e. temperature, top_p, ... but not the model deployment, that is treated as a seperate concern.