feat: serve all LLMs as OpenAI-compatible API

[sigoden]

$ aichat --serve
Access the API server at: http://0.0.0.0:8080/v1/chat/completions

curl http://127.0.0.1:8080/v1/chat/completions \
  -i -X POST \
  -H "Content-Type: application/json" \
  -d '{
    "model": "openai:gpt-3.5-turbo",
    "messages": [
      {
        "role": "user",
        "content": "Hello!"
      }
    ],
    "stream": true
  }'

Note: Users can use default model name to select currently active model.