A single exchange rate entry.
Rates coming from tables A and B carry a :mid (average) value, while
table C rates carry :bid/:ask values. Fields that are absent in the
API response are nil.
When the rate is part of a currency time series (NbpReq.rates/3), the
:no (table number) and :effective_date fields are set instead of
:currency/:code, which then live on the parent NbpReq.CurrencyRates.