Refactor: Improve type safety

[AAkira]

This PR introduces type-safe refactoring across the codebase and reorganizes platform-specific parameters for better maintainability and developer
experience.

Key Changes

Type Safety Improvements:

• Refactored CallEvent to use sealed classes with pattern matching for exhaustive event handling
• Added toString() methods to all entity classes for better debugging
• Changed activeCalls() return type from List<dynamic> to List<CallKitParams> for type safety
• Updated example code to leverage type-safe APIs with pattern matching

Platform Parameter Reorganization:

• Android: Consolidated Android-specific parameters (isCustomNotification, logoUrl, ringtonePath, backgroundColor, backgroundUrl,
  actionColor, textColor, etc.) into AndroidParams
• iOS: Organized iOS-specific parameters (iconName, handleType, normalHandle, audioSessionMode, etc.) into IOSParams
• Moved normalHandle from common parameters to iOS-specific (as it's only used on iOS)
• Removed deprecated uuid field (now using id consistently)
• Moved textAccept and textDecline to platform-specific parameters
• Added headers parameter support
• Reorganized parameter order in native code for better readability

Other Improvements:

• Updated Dart SDK version
• Fixed example application to use new type-safe APIs
• Improved code organization and readability

Breaking Changes

• CallEvent now uses sealed classes - event handling code needs to be updated to use pattern matching
• activeCalls() now returns List<CallKitParams> instead of List<dynamic>
• Some parameters have been moved to platform-specific classes (AndroidParams and IOSParams)

Migration Guide

Event Handling:

// Before                                                                                                                                         
switch (event!.event) {                                                                                                                           
  case Event.actionCallIncoming:                                                                                                                  
    // handle                                                                                                                                     
    break;                                                                                                                                        
}                                                                                                                                                 
                                                                                                                                                  
// After                                                                                                                                          
switch (event) {                                                                                                                                  
  case CallEventActionCallIncoming():                                                                                                             
    // handle                                                                                                                                     
    break;                                                                                                                                        
}                                                                                                                                                 

Active Calls:

// Before                                                                                                                                         
var calls = await FlutterCallkitIncoming.activeCalls();                                                                                           
if (calls is List && calls.isNotEmpty) {                                                                                                          
  var id = calls[0]['id'];                                                                                                                        
}                                                                                                                                                 
                                                                                                                                                  
// After                                                                                                                                          
var calls = await FlutterCallkitIncoming.activeCalls();                                                                                           
if (calls.isNotEmpty) {                                                                                                                           
  var id = calls[0].id;                                                                                                                           
}                                                                                                                                                 

Test Plan

Please carefully confirm all changes.

• Verify incoming calls work on both Android and iOS
• Verify outgoing calls work on both Android and iOS
• Test call accept/decline actions
• Test missed call notifications
• Test custom notifications on Android
• Verify iOS CallKit integration
• Test all callback events with pattern matching
• Verify example app works correctly