feat: idr rate aggregator

[fauziladzuardhirokhmana]

Summary

This PR implements an IDR rate aggregator service that fetches:

• Latest IDR exchange rates
• Historical IDR to USD rates
• Supported currencies

The system uses a strategy pattern to handle different data sources and ensures consistent response formatting using DTOs.

Features

• Fetch latest IDR exchange rates with spread calculation
• Fetch historical IDR to USD exchange rates
• Fetch supported currencies
• Consistent JSON response using DTOs
• Error handling for null or invalid API responses

The Strategy Pattern is used to decouple different data-fetching logics.

Each data type (latest rates, historical rates, supported currencies) has its own implementation of the IDRDataFetcher interface.

This makes the system:

• Easily extendable (new data sources can be added without modifying existing code)
• More maintainable
• Cleaner separation of concerns

DTOs are used to define a clear contract between the backend and clients.

Benefits:

• Avoid exposing raw API responses
• Ensure consistent and controlled JSON output
• Easier to maintain and modify response structure

Error handling is implemented at the fetcher level.

• Null responses from external APIs are validated
• Exceptions are logged using SLF4J
• Runtime exceptions are thrown with meaningful messages

This ensures failures are detected early and are easier to debug.