.Net Enable Usage of Custom Compatible Chat Message API Endpoints with OpenAI Connector + Examples

[RogerBarreto]

Motivation and Context

Resolves #5353

• Allow usage of custom Message API (OpenAI ChatCompletion Standard) compliant endpoints with the OpenAI Connector.
• Refactoring of OpenAI Models and Classes Structure
• Adding Examples on using the current changes against LMStudio, Ollama and LocalAI Message APIs.

[Krzysztof318]

I have never used LM-studio, but what this has common with the openai implementation?

[stephentoub]

I assume this is temporary and having someone implement their own handlers and extension methods and whatnot isn't actually our answer for this, right?

[RogerBarreto]

As stated in the Connectors ADR, while the biggest benefit of LM Studio is mimicking OpenAI's Endpoints and Contracts no specific Connector for this platform is needed, unless we have a specific requirement that the OpenAI connector isn't enough for consuming LM Studio Http APIs, we will suggest using the OpenAI's Connector.

We will add an extra configuration option for the OpenAI connectors that won't require to manually define an Handler for it and investigate if the usage of the Endpoint configuration would alone suffice for this.

[matthewbolanos]

@RogerBarreto, we should have a better 1st class story for connecting to popular local deployment channels like LLM Studio and Ollama, but in the meantime, I think your sample would be valuable for people who need to customize the OpenAI API endpoint.

Could you update this PR to make it more "generic" (i.e., title the Kernel Syntax Example "custom OpenAI endpoint") so we can get this merged in?