[SAMPLE] Microsoft.Extensions.AI function calling Sample

Couple things that need to happen for this to work:

• Update ORTChatClient to support generic function calling, similar to the Ollama client. The client just needs to know function call messages exist and how to add them to the history and only return message output, but the FunctionInvokingChatClient extension should handle function invocation automatically.
• Update prompt formatter to handle function calling cases. This will probably require per-model templates to make sure each model is getting available functions in the format it expects.

@Jaylyn-Barbee will support me on this since this should help enable Agent samples.