Implement Synth as an exchange rate provider (#574)

* Implement Synth as an exchange rate provider

* Add assertions to provider interface test

* Assert the correct provider error is raised

* Remove unnecessary parens

app/models/exchange_rate/provided.rb

@@ -0,0 +1,24 @@
+module ExchangeRate::Provided
+  extend ActiveSupport::Concern
+  include Providable
+
+  class_methods do
+    private
+      def fetch_rate_from_provider(from:, to:, date:)
+        response = exchange_rates_provider.fetch_exchange_rate \
+          from: Money::Currency.new(from).iso_code,
+          to: Money::Currency.new(to).iso_code,
+          date: date
+
+        if response.success?
+          ExchangeRate.new \
+            base_currency: from,
+            converted_currency: to,
+            rate: response.rate,
+            date: date
+        else
+          raise response.error
+        end
+      end
+  end
+end