Update OpenAI client configuration and methods

[zurawiki]

Fixes and closes #202

Test Plan:

gptcommit config delete openai.api_base
gptcommit config delete openai.api_key
# ensure environment variables are clear
# run githook, should fail

# trailing slash is crucial because that is not the default API_BASE, but will still work without setting up your own custom base url.
gptcommit config set openai.api_base "https://api.openai.com/v1/"
gptcommit config set openai.api_key "sk-..."
# ensure environment variables are clear
# run githook, should pass and use api_key given