Set OPENAI_API_KEY env var to play around with it

This means you can now use any model for completion that llama.cpp can run

Just compile llama.cpp and run the server like this:

./server -m codellama-7b.Q5_K_M.gguf -c 4096

I've tested this with codellama 7B quantised (codellama-7b.Q5_K_M.gguf) and it works well. But I have no idea if the special `/infill` endpoint works for other models, as I don't know how llama.cpp would know about the infilling tokens

- Refactored the way copilot completion backends work
- Added Livebook.Copilot.BumblebeeBackend (including attempting to run Serving under new DynamicSupervisor)
- Added Livebook.Copilot.DummyBackend for testing
- Added Livebook.Copilot.LlamaCppHttpBackend for running models in llama.cpp's server locally
- Added Livebook.Copilot.OpenaiBackend for running on OpenAi
- Added Livebook.Copilot.HuggingfaceBackend to use HF inference endpoints
- Played around with adding some user feedback via flash messages
- Fixed a whole bunch of edge cases and bugs in client side logic
- Request completions instantly (instead of debounced) when manually requested
- Added special comments you can put in livebook cells that will override the configured copilot backend

Reverting to GPT2