Add Ollama API support (Copilot-like with local LLMs)

[chhu]

Hi, I was recently trying VS code with the Continue Plugin, configured to use my own OLLAMA server and LLMs (https://ollama.ai) and was amazed how well this works.

I'm not a big fan of shipping my code to companies to train the models they sell, so I'm not a fan of copilot and the like. But with the option of local open-source LLMs this becomes a game changer.
Related to #20632