Add JSON Schema validation support

[simbo1905]

Add JSON Schema validation functionality to the project.

What: Implement JSON Schema Draft 2020-12 validation support as a separate module.

Why: To provide standards-compliant JSON Schema validation capabilities for Java applications using the JSON API.

The implementation should include:

• JSON Schema Draft 2020-12 subset support
• Thread-safe compiled schemas
• Immutable validation results with error paths/messages
• Integration with the official JSON Schema Test Suite
• Maven verify integration
• Documentation and usage examples

Scope: This covers the json-java21-schema module with validation features, test suite integration, and documentation.

[simbo1905]

Implementation completed in PR #18. The JSON Schema validation module is ready for review.