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

test/interfaces/exchange_rate_provider_interface_test.rb

@@ -0,0 +1,27 @@
+require "test_helper"
+
+module ExchangeRateProviderInterfaceTest
+  extend ActiveSupport::Testing::Declarative
+
+  test "exchange rate provider interface" do
+    assert_respond_to @subject, :fetch_exchange_rate
+  end
+
+  test "exchange rate provider response contract" do
+    accounting_for_http_calls do
+      response = @subject.fetch_exchange_rate from: "USD", to: "MXN", date: Date.current
+
+      assert_respond_to response, :rate
+      assert_respond_to response, :success?
+      assert_respond_to response, :error
+      assert_respond_to response, :raw_response
+    end
+  end
+
+  private
+    def accounting_for_http_calls
+      VCR.use_cassette "synth_exchange_rate" do
+        yield
+      end
+    end
+end