add uri_base support to Regent::LLM::OpenAI

[danielpuglisi]

This makes it possible to use OpenAI compatible APIs like Groq, Deepseek and also works with Ollama through the ruby-openai gem: https://github.com/alexrudall/ruby-openai?tab=readme-ov-file#azure

Regent::LLM::OpenAI.new(
  model: 'qwen-qwq-32b',
  api_key: 'groq_api_key',
  uri_base: 'https://api.groq.com/openai'
)