Introduces a new API key interface that allows users to customize how
API keys are provided and managed in their Spring AI applications. This
change improves security and flexibility by:
- Adding core ApiKey interface and SimpleApiKey implementation
- Adding builder pattern for OpenAiApi creation
- Deprecating public constructors in favor of builder API (since 1.0.0.M6)
- Added docs
The new system enables users to implement their own key management
strategies while maintaining backward compatibility with property-based
configuration.