This module is responsible for defining the behaviour of the feed (HTTP) client.
@callback get_historic_rates(options :: keyword()) :: {:ok, binary()} | {:error, term()}
@callback get_last_ninety_days_rates(options :: keyword()) :: {:ok, binary()} | {:error, term()}
@callback get_latest_rates(options :: keyword()) :: {:ok, binary()} | {:error, term()}