Support a local Ollama client

[jpmcb]

The Ollama project is an agnostic runtime for LLMs that runs models in containers: https://github.com/ollama/ollama this can be run locally or within a cloud environment.

I'd love to use gptscript, but I have no incentive to use openAI's gpt 3.5 or more expensive gpt 4 (while it's pretty cheap with lite use, over time, this could easily add up to $100s of dollars) when I have my own hardware and GPUs that I could run with codellama, llama 2, Mistral, or any of the other models the ysupport.

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Proposal:

1. Refactor the pkg/openai package to be an agnostic type interface in pkg/client:

https://github.com/gptscript-ai/gptscript/blob/f162c5aa3f7971309d4a4347360fd43fa3e7c497/pkg/openai/client.go#L35-L40

2. Implement an Ollama client that sets the client.url to some env var provided by the user (in the local environment use case, this would likely be localhost)

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I'd be happy to try and take a stab at this one since I have experience building local ollama integrations with neovim: https://github.com/jpmcb/nvim-llama