Update Ollama implementation

Currently we treat the Ollama native API as if it was the OpenAI compatible API. We should distinguish and gives users the option between switching between the Ollama native, OpenAI, or Anthropic compatible APIs.

Inspired by this message from Erik Tews: There is one thing that is a bit surprising for me. Rig has a OllamaCompletionRequest with a max_tokens element, but apparently in Ollama there is no such option in the API: https://github.com/ollama/ollama/blob/main/docs/api.md