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/concerns/providable.rb

@@ -0,0 +1,13 @@
+# `Providable` serves as an extension point for integrating multiple providers.
+# For an example of a multi-provider, multi-concept implementation,
+# see: https://github.com/maybe-finance/maybe/pull/561
+
+module Providable
+  extend ActiveSupport::Concern
+
+  class_methods do
+    def exchange_rates_provider
+      Provider::Synth.new
+    end
+  end
+end