feat: implement IDR rate aggregator

[ramaaufadha]

📌 Overview

This PR implements a backend service to provide IDR-based financial data using the Frankfurter API.

The solution follows clean architecture principles and applies several design patterns to ensure scalability, maintainability, and performance.

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⚙️ Key Features

• Strategy Pattern to eliminate conditional logic
• FactoryBean for WebClient configuration
• In-memory caching with preloaded data
• Global exception handling
• Unit testing

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🧠 Architectural Rationale

1. Why Strategy Pattern?

Strategy Pattern is used to avoid conditional logic (if/switch) and make the system easily extensible. New resource types can be added without modifying existing code.

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2. Why FactoryBean?

FactoryBean encapsulates WebClient creation logic, making configuration centralized, reusable, and more flexible.

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3. Why preload data at startup?

Data is fetched once at application startup to improve performance, reduce external API calls, and ensure faster response times.

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🧪 Testing

• Unit tests implemented for SpreadCalculator
• Basic validation for strategy logic

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💡 Notes

• All endpoints are working correctly
• Data is stored in memory using immutable structure
• Designed for scalability and maintainability