feat: Add Credentials struct for simplified client initialization

[echozyr2001]

This PR introduces a new Credentials struct that consolidates API key and base URL
configuration, enabling simpler client initialization. The main benefits are:

• Single container for auth and URL settings
• Environment-based configuration in one step via OpenAI::from_env()
• Cleaner credential management with OpenAI::from_credentials()
• Full backward compatibility with existing code

Environment variables:

• OPENAI_API_KEY (required): Your API key
• OPENAI_API_URL (optional): API base URL, defaults to "https://api.openai.com/v1/"
• OPENAI_ORGANIZATION (optional): Organization ID

This change improves the developer experience while maintaining all existing functionality.