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

lib/retryable.rb

@@ -0,0 +1,19 @@
+module Retryable
+  def retrying(retryable_errors = [], max_retries: 3)
+    attempts = 0
+
+    begin
+      on_last_attempt = attempts == max_retries - 1
+
+      yield on_last_attempt
+    rescue *retryable_errors => e
+      attempts += 1
+
+      if attempts < max_retries
+        retry
+      else
+        raise e
+      end
+    end
+  end
+end