feat(examples): Add comprehensive SDK examples addressing Issue #24

[Kellytomi]

What This PR Does

I've created a comprehensive example package that directly addresses all requirements outlined in GitHub Issue #24 - Add Comprehensive Example Usage for MindPaystack SDK. This provides developers with the exact end-to-end examples requested in the issue.

🎯 Problem Statement (from Issue #24)

The issue identified that there was "no end-to-end example demonstrating how to use the MindPaystack SDK" and requested comprehensive coverage of SDK initialization, payment methods, error handling, and best practices.

✅ Complete Issue Requirements Coverage

1. SDK Initialization ✅

• ✅ How to import and configure MindPaystack
• ✅ Setting up PaystackConfig with public/secret keys and environment
• Our solution: Comprehensive config examples with test/production patterns

2. Charging a Card ✅

• ✅ Step-by-step: collect card details, create charge, handle authorization/OTP
• Our solution: Complete card payment flow with authentication handling

3. Listing and Managing Payment Methods ✅

• ✅ How to list, add, and remove payment methods for a customer
• Our solution: Full payment method management examples

4. Transaction Operations ✅

• ✅ How to initiate, verify, and refund transactions
• ✅ How to fetch transaction history
• Our solution: Complete transaction lifecycle examples

5. Using Payment Channels ✅

• ✅ How to retrieve available payment channels and their fees
• Our solution: Bank listing, fee calculation, and channel information

6. Error Handling ✅

• ✅ How to catch and interpret MindException and other errors
• Our solution: Comprehensive error handling patterns and retry strategies

7. Best Practices ✅

• ✅ Recommendations for secure key management
• ✅ Notes on handling sensitive data (PCI compliance, etc.)
• Our solution: Complete security guide with do's and don'ts

8. Code Comments and Documentation ✅

• ✅ Inline comments explaining each step
• ✅ Links to relevant API docs
• Our solution: 780+ lines of documented examples plus comprehensive markdown guide

✨ What We've Delivered (Beyond Requirements)

📦 Files Created:

• comprehensive_example.dart: 780+ lines covering all SDK features
• EXAMPLES.md: Complete documentation with setup guides
• Enhanced CLI: Interactive mode with help system

🚀 Additional Value:

• Interactive CLI interface for choosing examples
• Production-ready integration patterns
• Security best practices throughout
• Real-world error handling examples
• Complete testing strategies

🛡️ Security & Best Practices

Addresses the security concerns mentioned in the issue:

• API key management: Environment variables, rotation policies
• PCI compliance: Card data handling guidelines
• Error handling: User-friendly messages, logging best practices
• Webhook verification: Signature validation examples

🧪 Example Structure Match

The issue provided this example structure:

await MindPaystack.init(PaystackConfig(...));
final chargeResponse = await MindPaystack.instance.charge.chargeCard(...);

Our comprehensive example includes this exact pattern plus:

• Complete error handling around it
• Multiple payment methods (not just cards)
• Transaction verification flows
• Payment method management
• Production deployment patterns

📋 Acceptance Criteria ✅

From Issue #24:

• ✅ Example covers all major SDK features
• ✅ Includes error handling and best practices
• ✅ Well-commented and easy to follow
• ✅ Ready to be linked from main README

🎉 Issue Resolution

This PR completely resolves Issue #24 by providing:

1. Everything requested in the original issue
2. Additional value with interactive CLI and comprehensive docs
3. Production-ready examples that developers can use immediately
4. Security best practices for real-world deployments

🔧 Usage

# Interactive mode (choose which example to run)
dart run bin/sample_payment_dart.dart

# Run comprehensive example directly  
dart run bin/sample_payment_dart.dart comprehensive

# Get help
dart run bin/sample_payment_dart.dart help

---

Resolves: #24
Created by: Kelvin.dart
Documentation: See apps/sample_payment_dart/EXAMPLES.md

[Yczar]

Hi @Kellytomi , thank you so much for your patience and for being excited to contribute!

@developerjamiu and I have been carefully shaping the direction of this package and mapping out the roadmap we believe will make it even stronger. Through this process, we’ve restructured a few things and introduced a fresh approach for contributions and the example project.

It would be amazing if you could take a look at the latest updates and the contributors’ guide; this way, your input can directly help shape where the project goes next. We’d really value your perspective as we build this together!