Support macOS 10.15 and added a new Obj-C wrapper

[guidedways]

• [Y ] All new/changed/fixed functionality is covered by tests (or N/A)
• [ Y] I have added documentation for all new/changed functionality (or N/A)

📋 Changes

• Changed macOS target to 10.15+ (as it's supported without any code changes)
• Obj-C support: added new SimpleKeychainObjC class that wraps the Swift SimpleKeychain struct
• Used conditional compilation to support LAContext on platforms where unavailable

Provides the same functionality as the Swift API but with Objective-C conventions

🎯 Testing

• Added new unit tests