Modernized constructors and added return types for PHP compatibility

[atillavanilla]

What Changed

• Updated constructors in Api, Client, and Model files (including Configuration.php) with ?array $data = null to fix PHP deprecation warnings.
• Added return types (bool, mixed, void) to improve type safety and compatibility.

Why It’s Needed

• Fixes PHP deprecation warnings and ensures compatibility with modern PHP versions.
• Enhances code clarity and maintainability.

Considerations

• Fully backward-compatible with PHP versions that support nullable types and return type declarations.
• Minimum PHP version requirements may need updating.