Exchange Rates

Spree stores one explicit amount per currency everywhere money is configured and never converts between them. That stays. This adds an ExchangeRateProvider that fills fields in, and nothing else: a merchant types an amount in one currency and the dashboard offers editable suggestions in the others. What is persisted is a plain number the merchant owns, and the rate feed is never consulted again for it. Rates suggest, they never compute a charge.

Plan: https://github.com/spree/spree/blob/main/docs/plans/6.1-exchange-rates.md

Tracked in Linear as V-3614.