ExchangeRateProvider

A trait for providing exchange rates.

This is the central SPI (Service Provider Interface) for currency conversion. Users can implement this trait to fetch exchange rates from any source, such as a web API, a database, or a local file.

Well-behaved implementations should handle finding direct rates (e.g., EUR -> USD), inverse rates (if only USD -> EUR is available), and potentially chained or cross-rates (e.g., KES -> ZAR via USD).