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/provider/synth.rb

@@ -0,0 +1,51 @@
+class Provider::Synth
+  include Retryable
+
+  def initialize(api_key = ENV["SYNTH_API_KEY"])
+    @api_key = api_key || ENV["SYNTH_API_KEY"]
+  end
+
+  def fetch_exchange_rate(from:, to:, date:)
+    retrying Provider::Base.known_transient_errors do |on_last_attempt|
+      response = Faraday.get("#{base_url}/rates/historical") do |req|
+        req.headers["Authorization"] = "Bearer #{api_key}"
+        req.params["date"] = date.to_s
+        req.params["from"] = from
+        req.params["to"] = to
+      end
+
+      if response.success?
+        ExchangeRateResponse.new \
+          rate: JSON.parse(response.body).dig("data", "rates", to),
+          success?: true,
+          raw_response: response
+      else
+        if on_last_attempt
+          ExchangeRateResponse.new \
+            success?: false,
+            error: build_error(response),
+            raw_response: response
+        else
+          raise build_error(response)
+        end
+      end
+    end
+  end
+
+  private
+    attr_reader :api_key
+
+    ExchangeRateResponse = Struct.new :rate, :success?, :error, :raw_response, keyword_init: true
+
+    def base_url
+      "https://api.synthfinance.com"
+    end
+
+    def build_error(response)
+      Provider::Base::ProviderError.new(<<~ERROR)
+        Failed to fetch exchange rate from #{self.class}
+          Status: #{response.status}
+          Body: #{response.body.inspect}
+      ERROR
+    end
+end