feat: Create parent aggregator POM for API/SDK separation

- Convert single module to multi-module Maven project
- Setup dependency management for coordinated versioning
- Add modules: openfeature-api and openfeature-sdk
- Maintain backward compatibility structure
- Version bumped to 2.0.0 for major architectural change

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <[email protected]>

Author: aepfli