Add handling of type filters and handling of ETags

Add API documentation and test cases around ETags for the API.

app/controllers/repp/v1/retained_domains_controller.rb

@@ -2,10 +2,14 @@ module Repp
   module V1
     class RetainedDomainsController < ActionController::API
       def index
-        domains = RetainedDomains.new
+        domains = RetainedDomains.new(query_params)
 
         render json: { count: domains.count, domains: domains.to_jsonable }
       end
+
+      def query_params
+        params.permit(:type)
+      end
     end
   end
 end